There is no such programs that convert ds games to gba ones and vice versa.
There is such a program, if you had read my post I explained that I had one in the past.
What I mean is not to actually play nds rom on the gba.
The thing about this problem that rocks is that you can convert some very important homebrew files into .gba. Like flashme for example. Why would I need flashme as a .gba file? Because I use an EFA linker and the way their loader works is a little farked.
You can load gba games by selecting the "gba loader" which will give you a
list of the roms, or you can do the same with nds games by chosing the "nds loader".
Or, you can choose no loader and have your games boot directly with out without the list screen. It directly boots the game.
Now that you know about the loader options, ill tell you why the last one sucks (hard). For what every reason, they decided that .nds roms would not be supported without chosing the "nds loader". (I'm sure it was just a technical over sight, but its one that they have yet to fix since adding the nds support.)
So, I need the flashme.gba file to be able to flash my DS. And yes this method works because its how I flashed my first DS. My newer DS has a different firmware and I have been too lazy to flash it / I've had to format and don't have the correct files.
Edit: Thanks Thug4L1f3, I remember you from when I use to frequent the forums