To Answer this question just need to look in the Bestiary (p. 298) under the Universal Monster Rules.
Change Shape
A creature with this special quality has the ability to assume the appearance of a specific creature or type of creature (usually a humanoid), but retains most of its own physical qualities. The creature cannot change shape to a form more than one size category smaller or larger than its original form. This ability functions as a polymorph spell, the type of which is listed in the creature’s description, but the creature does not adjust its ability scores (although it gains any other abilities of the creature it mimics). Unless otherwise stated, it can remain in an alternate form indefinitely. Some creatures, such as lycanthropes, can transform into unique forms with special modifiers and abilities. These creatures do adjust their ability scores, as noted in their descriptions.
Format: change shape (wolf, beast shape I); Location: SQ, and in Special Abilities for creatures with a unique listing.
As you can see it specifically states here that change shape is duration is indefinite, and the entry under Kitsune does not counter this.
Tome of Magic has Empower Supernatural Ability, Enlarge Supernatural Ability, Extend Supernatural Ability, and Widen Supernatural Ability. They have the effects of the metamagic feats of the same name, except that they work on Su abilities, once per day per time you take the feat.
Races of the Dragon also has a handful of breath feats that work for dragonfire adepts. Unlike metabreath feats, which require a breath weapon that has a cooldown “expressed in rounds,” breath feats merely require that you have and use a breath weapon to activate them. Entangling Exhalation is by far the best of these feats; the others are Exhaled Barrier, Exhaled Immunity, Extra Exhalation, and Furious Inhalation.
Finally, note that while Draconomicon’s metabreath feat requirement is having a breath weapon with a cooldown expressed in rounds, their benefit can apply to any breath weapon. Thus, if a dragonfire adept has some other breath weapon (from the dragonborn template from Races of the Dragon, perhaps), he qualifies for metabreath feats, which may be applied to his dragonfire adept breath weapon. This would add a cooldown to the dragonfire adept’s breath weapon when used this way, however. Plus, ultimately, few of the metabreath feats are all that impressive; I probably wouldn’t bother with the hoops you have to jump through to do this.
I strongly recommend Entangling Exhalation for all dragonfire adepts. The rest of these feats are fairly mediocre.
Best Answer
Turning a single spell, or limited number of spells, into a spell-like ability or supernatural ability is definitely possible.
For spell-like ability, the most immediate example is the archmage, which has an option for its high arcana class feature simply called “Spell-like Ability.” You choose a spell, permanently lose one 5th-level spell per day as well as one spell per day of the chosen spell’s level, and you get to use that spell 2/day as a spell-like ability. This results in basically trading a 5th-level spell slot for a higher level spell slot, but you lose the flexibility of having possibly prepared something else in those slots.
There are other ways to get spell-like ability versions of selected spells; too many to list here.
Supernatural spells are rarer. The only example to come to mind is the dweomerkeeper prestige class from the web enhancement to Complete Divine. This is generally considered an overpowered class because of its ability to turn any spell into a supernatural ability, since supernatural abilities have significant advantages over spells. In particular, unlike the archmage, dweomerkeeper neglects to say you still have to pay material and XP components on the chosen spell, which means you can use, for example, wish as a supernatural ability that doesn’t require XP.
There is no way to turn all of a chosen class’s spellcasting to spell-like or supernatural. However, one particular class does this naturally: the shadowcaster from Tome of Magic. There are a lot of things going on with the shadowcaster, which I’ll skip, but the long and short of it is that the shadowcaster uses “mysteries,” which start out as arcane spells. When the shadowcaster gets 4th-level mysteries, his 3rd-level and lower mysteries start to count as spell-like abilities (his new 4th-level mysteries are still cast as arcane spells). Then when he gets 7th-level mysteries, his 3rd-level and lower mysteries start to be used as supernatural abilities (and his 4th through 6th-level mysteries start to be spell-like, and his new 7th-level mysteries are still arcane).