Hi Andrew, the term "Loft" in MoI means to construct a surface between 2 or more cross section curves, as shown here:
http://moi3d.com/3.0/docs/moi_command_reference7.htm#loft

If I understand your question correctly, you are asking about converting 2D curves into solids by what in MoI is called Extrude, as in here:
http://moi3d.com/3.0/docs/moi_command_reference7.htm#extrude

So to put your question in terms used by MoI, I think you're asking if MoI is able to directly extrude a solid box from a line segment. The answer to that is no - you would need to make a rectangle curve first to extrude to a box. You can use the Construct > Offset command to generate a rectangle from a line segment though, and then extrude that to a box.

For generating text as others mention above it's easiest to use Draw solid > Text will generate solid text from a font.

- Michael