On CO and NW, I've done this, right up to 364 days.

I used an outbound award to position in the EU and when after about fall 2008, supercheap ex-EU MR or FD type itins from EU to US appeared from time to time, I'd buy another and bracket it with the return leg of the award. In the end, I pushed the return leg back six times (no fees remember, unlike DL, making this a very fruitful exercise). I would have done it again but I was reminded by the CSR that for the last time, I was using it on the very last day of validity and it was going to be use it or lose it.

This is unlike a revenue ticket which can be reissued and - on CO at least - will show a new expiration date corresponding to a year from reissue - you're out of luck if you want more time on an award.