If you on the Objects folder>Inport>Prop you can import a polygonal model as a "Prop" This will be a solid shape not editable in A:M but usable in chors.
If you open a new blank model window then >plugins>Import>3D Studio you can load a polygonal model and A:M will try to convert the facet edges to splines. This may be slow. This is editable, however, the chances that the polygonal model was created with the same arrangement of CPs that a good A:M model needs is nearly zero.
Polygonal models made of triangles are hopeless cases.
In general, most polygonal models are poor candidates for import to A:M.