Ok, do you know how to use plugin2GBA for these programs? Also, if I convert the NES, and Gameboy games using these emulators for the GBA, do you know if the save files be usable with other emulators?
Save files are usually simple enough to transport between things.
Most emulators on anything will use raw save files, that is to say the file as it appears on your hard drive/SD card/whatever will be that which the original hardware has. Accordingly these can be transferred around the place, and even if not then many emulators will have import options (vba called it battery save, others might have raw import options)
If the save is huge them sometimes things will compress them to save space. Not really the case for NES and GB stuff. We did see it for some DS stuff, especially where some flash carts defaulted to using a massive save file compared to what would actually have been there. Does not apply to GBA.
Some dumping tools, and even fewer flash carts/flash cart handling programs, will add a header or footer usually to note what game it belongs to*, or maybe some other data like real time clock position. In which case figure out what is header and footer (it will usually be fairly obvious if you start the game, save and compare things, and most saves are some power of 2 in size where this will be power of two plus or minus something. You might even find someone made a "converter" program before you as it is a reasonably common activity.
*more modern stuff that has say an OS in the background will note what belongs to what, however a game cartridge has its save onboard so does not care/need to know what the save belongs to, the original devs were presumably working on at most two games so don't care to implement such things/waste valuable save space and there is little other compelling reason to note what save belongs to what outside of a flash cart or save dumping tool that might have 500 saves and maybe lose the names somehow.
Some emulators if they have multiple games within their own menu structure (which could include these GBA emulators and often did, especially as most GBA cartridges would not have had a file system to play with like was standard for much of the DS to be able to drop individual saves for games even if packed together) will have saves packed in there in a primitive archive type arrangement. If you can find its location within the file (start a game, compare and see if anything matches, the save format might even tell you if you know what you are doing) you can slice it out easily enough.
Its gets a bit more tricky with savestates where different emulators might put things in a different order, have different padding between sections, might have to miss sections (some things might be read only), employ some compression or similar but ultimately a savestate is a collection of memory dumps so if push comes to shove you can still get stuff done.