Though I wouldn't do it, I don't think small tack welds would hurt anything (except your warranty). You're not getting the metal hot all the way through where it would lose its temper. I would not make any major welds on it though. The same applies to (for example) welding on truck frames. They are heat treated and getting them too hot can weaken them.
Again, I wouldn't do it. I'd just design some type of U-bolt bracket to attach to the
ROPS.