It is probably *possible* in ICE, but it is MUCH more tedious than if you were to do it in IceCC. Let me elaborate just a little bit:
Like Voyager said, in IceCC, all the units have their own offsets for their animations. The Marine's ground attack will point to MarineGrndAttack (uh, or something similar), and I'm assuming the marine's air attack will aslo point to MarineGrndAttack. But the important thing is that NO OTHER UNIT will point to MarineGrndAttack, even if the animation script in there is exactly the same. This means you can happily mash up that script and make it grind starcraft to a palette-inversing crash, and units with similar attacks like the Hydralisk will not be affected.
In ICE, the marine's ground attack will point to some offset number like 56781(this number is made up), and it is not obvious how many other things are pointing to that, and it's risky to go and edit that. I also find the interface a pain to use in that you have to go search up the offsets, make your changes, go back, check another offset, search that one, make changes, and so on. Don't use ICE
Now, to do what you want to do involves adding another animation script, which is easy to do it iceCC, since you can just add another identifier and make the marine refer to it directly, or jump to it with the opcode Voyager mentionned. In ICE, you'd have to juggle numbers. And I'm not entirely sure, but I think you'd have to find an offset number to edit that's not being used by anything else to make this new script thingy. Don't use ICE
As for your technical problem: For IceCC, what I usually do is decompile the WHOLE iscript.bin to iscript.txt, and then work with that, instead of decompiling parts of the script and then merging them with the default iscript.bin again. Maybe that would work better on your system? Try to use the IceCC User Interface and decompile the whole thing instead of one of the unit scripts alone. And then get to know the syntax for the iscript.txt file. If you've done some html, you should be able to conceptualize the whole thing and learn it really easily. I swear, it's not hard!
Don't use ICE