You can try to repair you mesh with some tools.
As i don't know your topology, let's use a simple cylinder with NGons as an example.
Here you have the views of the cylinder with a subsurf, smooth shaded and the rendered view.
1 : the supposed original mesh, with NGons
2 : the same mesh triangulated, as your imported mesh is supposed to be.
Now let's quadify it. Select all with A and then Alt+J. This might be enough to fix your mesh if there was no NGon in the original one.
3 : quadified mesh
If there was some NGons, select the face loop around the cylinder (possible with quads) with Ctrl+Alt+Right Click.
Invert the selection with Ctrl+I
Use F to make the faces NGons. NGons are supposed to be flat.
Now we're at step 4, or back to 1 : the original mesh.
Select your NGons, use I to inset, move your mouse to place your edges and left click. Do it again if necessary.
5 : repaired mesh