.STL is a polygon format

MoI can't import polygon data.
There is not specific polygon modeling tool in MoI either.

From the command reference :

MoI supports reading in curve and surface data from 3DM, IGES, STEP, and SAT files. 2D curves can be opened from PDF and AI files, and curves and drafting entities like lines, circles, and arcs can be opened from DXF files.

(rhino can store polygon in .3dm, but this data will be ignored in MoI)