Simple modify unit help plz.

[Tweety]

Hi.

I have been playing winspmbt for a while now but i havent figured out how to modify units.
I want to do just one single "simple" modify to one unit only.
Finnish sniper should have "hvy sniper" (m82) available just like in real life ( http://en.wikipedia.org/wiki/Barrett_M82 )
So i would want to modify my game to reflect this, if its not easily possible to "create a new unit for fins" its okay to me to just modify one of the later date sniper so that the gun it uses is m82.

Basicly question is how i can do it ?