Yes, you copy the file like it is, it's unlocked.
It's another type of "instalation". Not via MapSource.
If you connect your device at PC and look in Garmin folder, you'll see gmapsupp.img file that I presume it's Romania.
In that case there are 2 possibilities:
1. rename gmapsupp.img to gmapprom.img, and after that copy gmapsupp.img(Bulgaria).
2. combine the existing gmapsupp(Romania) with the other gmapsupp(Bulgaria) with GMapTool program, and after that you copy the resultant file(gmapsupp.img) on your device.
For me it's easy to speak, beacuse I have read a lot about this kind of stuf