Yes, obviously red storm used a modified IGOR version, but the .mis files remain mostly compatible.
So far I've found:
1. you can play on any .map file, which is a great first step.
2. Can't seem to add actors to a map, but you can add enemies to "defend" mode using assault platoons.
3. Some .wav files used in the grit_xbox maps cause IGOR to crash, so just delete all the sound files from the .mis file before loading it into IGOR. Then you can edit the map all day long. But sometimes the map gets messed up, so I usually just manually copy/paste my changes to the mis.
Oh, and the gr1 maps included for xbl play are fully playable in mission mode with a few edits.