I think you can 'trick' the audience in doing so.
1/ Order the SCV to attack the damaged building.
2/ Create a large net of triggers which 'heal' the building (e.g. make triggers which heal the building by 1% each mili-second - time can be modified)
3/ Once the building has 100% health, ally the SCV player with the building's player OR make the building invincible. (this will make the SCV stop 'repairing')
4/ Order the SCV to do another thing.
However, there are some problems which can interfere in what you want:
1/ The main problem I find is that both SCV and building must be from different players (you can't order a unit to attack a unit of the same player with triggers)
2/ Another problem is that the SCV's 'flame' is not exactly the same when it repairs than when it attacks (although people don't normally notice that)
3/ A smaller problem is that you're gonna have a hard time creating the trigger net and to test it so that it works (but once it works, you're done)
I think that's the closest thing you can get with normal triggers...
See ya!

RiCHaN