I don't see why not; the troublesome part, in my view, would be making the animation to go with it. Of course there would have to some compromises, but having created the model you could either configure it as:
A Proxy
Have it take up the launcher weapon slot and when a unit has one it would hang in front of him (like the RPG hangs on the back).
Problems:
would need to make sure that it's bullet-proof
would need scripting to make sure the unit doesn't do odd things, like lie down with it.
unless you define a new unit to carry it, with a whole new set of animations, it's not going to look as good as it might.
A Vehicle
The unit that carries the shield is now the driver in a vehicle, and the gun becomes a turret with limited traverse. This would need fewer animations, basically stand with shield and walk forward with shield. Each version in the config could only have one weapon to be used with it (when getting in the shield it would have a script to take magazines from the unit and load them into the vehicle).
Problems:
less manoeverable
might blow up when killed
unit doesn't have a choice of weapons to use with the shield.