No, you can't cast it using a spell slot.

Spells you can cast because of your race aren't spells you know, and thus can't be spells you prepare. Per the PHB (p. 201, "Known and Prepared Spells"):

Before a spellcaster can use a spell, he or she must have the spell firmly fixed in mind ...

... and it goes on to describe some of the class-specific variations of knowing and preparing spells. But in any case, having a feature from your race or class that allows you to cast a spell isn't the same as knowing the spell.

This is spelled out clearly in the multiclassing rules for Pact Magic quoted in the question (PHB p. 164; emphasis mine).

If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature, and you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know.

Likewise, the Drow Magic trait clearly distinguishes the known cantrip from the merely castable other spells (PHB, p. 24; bold for emphasis mine):

You know the dancing lights cantrip. When you reach 3rd level, you can cast the faerie fire spell once per day. When you reach 5th level, you can also cast the darkness spell once per day. Charisma is your spellcasting ability for these spells.

If the intent of the rules was that Drow also knew the non-cantrip spells, it would say so.

There certainly are options here.

• The Mother Cyst feat from Libris Mortis grants a series of spells known that revolve around necrotic cysts, several of which are relevant to you.

  The horrifying effectiveness of this feat led to its being the central, foundational pillar of Brian Bullsun-Stanton’s brutal magical surveillance state. That dip in urban druid could also be valuable—the susurrus of the city spell that is uniquely found on the urban druid list plays a big role as well.

  Specific highlights:

  • The 2nd-level necrotic cyst implants a necrotic cyst as an instantaneous effect (read: the only way to get rid of it is to surgically dig it out, if the target even knows it’s there). This, among other things, gives the target a −2 penalty on Necromancy spells, allows the rest of the spells from the feat to target this person, and prevents protection from evil et al. from blocking the other spells offered by the feat.

  • The 2nd-level necrotic scrying is a no-save scry on anyone who has a necrotic cyst. Never lose track of a cultist again.

  • The 4th-level necrotic domination works like dominate person except that it is Necromancy [Evil], not Enchantment [Mind-Affecting] (and it requires the target to have a cyst). This means it ignores mind-affecting immunity, cannot be blocked by protection from evil, and the subject takes that −2 penalty on the save.

  • Various other spells from the feat allow for extremely reliable punishments to be meted out to those who have cysts, which can provide a lot of influence and control even when not actively dominating someone.

• The Gatekeeper Initiate feat from Eberron Campaign Setting adds mind blank; the rest aren’t particularly stunning, but mind blank really is just that good.

• The Greensinger Initiate feat from Eberron Campaign Setting adds charm person, charm monster, and mass charm monster, among others.

• The rainbow servant prestige class from Complete Divine adds all cleric spells to your spell list. That requires 10 levels, though (note: the class is officially/technically full-casting, because the text says so and text trumps the 6/10 spellcasting listed on the table).

• Find a domain that has the spell(s) you want, and you will probably have little trouble getting it.

  • A domain staff from Complete Champion allows you to cast each spell of the given domain 1/day from your regular slots. No frills, and a bargain at 36,000 gp, but you can only attune to one at a time (done while preparing divine spells).

  • If you want more than one domain, or have a problem holding a staff, a domain icon from Faiths of Eberron allows you to burn a use of turn/rebuke undead in order to spontaneously swap a spell to the spell of the corresponding level from the domain that the icon represents. You can get turn undead from a single level of sacred exorcist from Complete Divine without losing any spellcasting. That level dip is a fairly significant cost, but once you’ve done it the domain icons themselves cost just 10,000 gp.

  • The Arcane Disciple feat adds the spells from one domain to the spell list of “one class [that has] arcane spellcasting ability.” With the Southern Magician feat from Races of Faerûn or the Alternate Source Spell feat from Dragon vol. 325, that could arguably be your druid class. (Note: if you go with this trick, it also opens up another of other options since arcanists have more prestige classes that do this kind of thing than priests do.)

  • The contemplative prestige class from Complete Divine is full-casting, and grants a bonus domain at 1st and again at 6th. Can be any domain appropriate to your faith.

  • The church inquisitor prestige class from Complete Divine grants the Inquisition Domain at 1st; stuff like zone of truth, detect thoughts, and discern lies might be quite useful.

  • The divine oracle prestige class from Complete Divine (noticing a theme?) grants the Oracle Domain at 1st; that one has a number of useful divinations.