Transform it in OBJ format with MeshLab for example http://meshlab.sourceforge.net/

Then import it inside Moi with _importOBJ by Max Smirnov http://moi.maxsm.net/media/files/ImportObj.v.1.5.2015.09.10.zip

<< By Michael Gibson ;)
Someone has written a script for importing ascii type (not binary type) STL files into MoI, it's on this thread here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=7046.1

But please keep in mind (you probably knew this already but just in case) that STL files contain triangle mesh data, which is a different type of data than the NURBS surface data that MoI is designed to work with at modeling time. So something like a sphere for example will be set up as possibly several thousand individual planes and not just one single smooth sphere surface and CAD data structures are not designed to hold a huge number of faceted pieces like that.