That is strange as I have imported a shield mesh and adjusted the size, then exported with no problem. It did not require a texture but it does require 2 sets of UVs, typically spherical. The second UV is just a copy of the 1st UV use to connect the tangents. This I have done in Softimage|XSI.
1st I saved the scene of the ship (MyShip.exp) then I save the same scene again (MyShipShield) so the ship model is in a new scene. Then I created a new Polygonmesh --> Sphere and made it slightly larger than the ship. Adjusted the points so the shield fit the ship better then applied a spherical UV, the applied another spherical UV. I did not need to flatten out the UVs because no texture was applied. Connected the tangent to the second UV then save the scene. Now I open explorer and delete the ship model and export to .mesh. Save the scene 1st so you can open again with the ship and the shield to rework the polys then resave the scene, delete the ship and export to mesh.
Hope this helps.
Creating the UV Map
Creating the Tangent Map