Are you talking Cadians, Catachan or other? Different kits have different bits, I can only speak for Cadians.
The only problem should be the left hand, since the gun can be cut away from the main hand and replaced with the new one very easily. Posing, leaving the guns held one handed can do the job easily, but you'd want some (most?) to be held two handed still I guess. Greenstuff can do that, although it's fiddly. You could try making a small mould to cast your own hands after spending the time making one fit well.
Ideally there'd be left hands that cup the gun, which you see with
SM bolter arms for example but I can't think of any
GW guard arms that do it. Eisenkern Stormtroopers have arms like that, as shown top left of
this sprue. Eisenkern are my answer to all guard questions though ; p Top right shows right hands that you sit the gun on top of, you'd cut the lasgun away from cadian arms in a similar fashion.
This shows how eisenkern bits work with Cadian bits.