This has come up on the Paizo boards before but as far as I am aware no one has finished their work.
Short answer is there's no easy way to do it, but you should be able to take the suggestions in the Conversion Guide for converting the core classes and apply it to the generic classes as presented on the d20 SRD.
The generic modifications (Hit Die, saves, etc) are going to be fairly trivial, the hard part is going to be selecting which class features you'd allow as "feats". If you are interested in going that route, I would suggest you start with some of the abilities from Fighter, Monk, Sorcerer (from the "default" bloodline), and Cleric and add some of the class features from those. But there's no "correct" way to do that. You also need to remember that Pathfinder has a tendency to ramp the power levels of all classes, bonus feats, class abilities, etc, so that needs to play a factor in your conversion.
Alternatively, and I almost prefer this suggestion, is to take the NPC classes presented in the Pathfinder Core book, and modify them to have PC BAB, Saves, and other class features. So, for example you'd be playing an Adept with a higher BAB, and better Saves (probably following the Cleric Progression), but with Adept abilities and spell lists. You'd have to expand their abilities further on if you plan on playing a long time like that...
So, yeah, no easy answer, hopefully this will help give you some ideas.
Home Campaign is code for "everything goes".
All the other options are supposed to limit choices to ones either permitted (as Lair Assault and Encounters seasons may have regulation determining selection options) or suggested (Dragonmarks do not appear in canon Forgotten Realms).
The only mechanical impact that I can see for selecting the Eberron setting, is that the background options are limited. For example, Forgotten Realms Player's Guide backgrounds do not appear as options (though Neverwinter ones do).
I would have expected that setting specific themes, such as the Dark Sun ones, classes such as the Bladesinger, deity choices such as Pelor, and a more complete set of setting specific backgrounds (such as the aforementioned Neverwinter ones) would also have been restricted, but it appears not.
Best Answer
Short answer: Yes, you can, but it's not really any easier than starting from the ground up and building a new character.
Long answer: While fourth editon D&D shares many setting conceits with 3.5 and earlier editions, the two are very different games in terms of how characters are constructed and the way challenges are addressed during play.
You might want to check out this question about the differences between the two games for a general overview. The accepted answer is pretty good.
So, about converting your character to 3.5... Well, it depends on exactly what you've built, but since 3.5rd and 4th edition make reasonably similar assumptions about setting, it shouldn't be too hard to convert your character's concept across.
However, as you may have noticed, there are a lot of differences between 4th edition and 3.5, most notably including different scales of leveling and a differing level of combat-centric-ness that will likely make it impossible to convert your character without making compromises.
As far as I know, there's no formal downgrade migration path available, so you'll have to do the conversion the old-fashioned way: Hit the 3.5 books and try to work out how your character would fit the system.
Unfortunately, I can't give you more specific advice than that without knowing more about your character. Could you possibly post your character sheet?