Are you sure Ultimate Unwrap imports .x models with smoothing intact? When I try it doesn't. Note that it may seem like the smoothing groups are intact if the smoothing boundaries also happen to coincide with seams in the texture mapping. DirectX models will import with the geometry broken (and thus hard edged) on texture boundaries, something which can be repaired easily with Weld Model.
If you're getting the look you want then sure, exporting to b3d will work.
If you're getting the look you want then sure, exporting to b3d will work.