Simply put - what is asked by the OP is just not possible.
Characters / skeletal meshes have no need for lightmap UVs, as such, you’d be hard pressed in finding anyone who includes a lightmap viable UV in their character meshes.
So, even if you go throguh the process one mesh at a time, and even if you attempt to have the engine generate a lightmap uv for you, chances are you just won’t get very good light baked results…
If light baking is the end goal, convering meshes in engine is probably not a viable solution…