It is one of the many save editors available for the pokemon series, and main one people use these days for DS games. Quite possible to open up saves used by games on emulators/flash carts, copy the relevant details and inject that back into either the save you ripped from your original cart or you can just write the save from the emulator/flash cart back to the original game if you would rather. Pokemon versions can be slightly trickier to rip and write games to -- the thing the pokewalker used was on the save bus so you either need a tool aware of the save bus (most of the general save dumper/restorer programs made during the early days of the DS will work on basically every game that is not pokemon or a couple of ones with ridiculously large saves) or to do a hardware mod to bypass the pokewalker. Not sure what the kids are suggesting for that these days, especially not if you have a 3ds as there is probably something nice there.
The GBA side of things. Yes you can bypass the issues with having to use pal park (flash carts can be tricky*, though you can use emulators these days) and instead recreate the GBA pokemon you would transfer across to the DS game save
*the DS game will reach out and try to touch the original Flash saves of the GBA originals, however GBA flash carts tended to be SRAM for saves which is a different protocol. You can modify the DS game to instead read the SRAM of a handful of common GBA flash carts (see cory1492's pokepatch) but that means a DS flash cart and then having to put the save back on the original DS game if you wanted to use the various efforts to in turn migrate up to 3ds titles (though you could use save editors there too).