This can be done only for the iPod supported files, right?
(for example the ogg files needs to be transcoded)

So it is needed to know which files iPod supports ("easy")...and when the DAP is an iPod (the music player is not always an iPod :P).

Or maybe...there is an easy solution: If the file to transfer is of the same format of the transcode format destination (TM) do nothing, maybe can work.

I guess, if you look at it that way.

Probably easier to implement would be some sort of "only move these file types: " with a drop down of the file types in your library?

Then you don't have to worry about what the destination DAP supports.

Moving to the iPod component.

I don't think this should be possible. If you're using your iPod as an iPod, then it supports MP3 or AAC. If your music isn't in that format, it needs to be transcoded.

If you're using your iPod as a mass storage device, it needs to show up not as an iPod, but as a mass storage player. 

What if I have other file type in my music library that my iPod doesn't play, and I either:

 * Don't want them on my iPod at all
 * Don't want to wait for them to transcode