Right now, PCS2 allows manipulating both subobjects and shields in a variety of ways, but doesn't offer any way of transitioning between them.
I'd suggest doing this like that:
In submodel tab, "Add to shield" button would add the selected submodel to the shield mesh, making it a part of the shield. It wouldn't be reversable except by ctrl-z, since it'd remove the submodel's textures and permanently merge it with the shield mesh.
In the shield tab, "convert to subobject" would convert the entire shield mesh to an untextured (or, for example, using the "invisible" texture) subobject.
I think it could be useful. For example, for making shields that stick very closely to the ship's surface, or shielding individual components, like turrets, fighterbays, etc. Right now, there's no way to do either without a modeling program.