Hi Andy,

> Is there a way to inset a particular surface while not extruding it?

Inset will always try to make a protrusion inwards or outwards, but if you enable the "Grooved" option and leave the groove width to be empty or set groove width to 0, you will get the "plug" as a separate object which will be flush to the original shape.

Or depending on what you want you may actually want to put in a small value for the groove width so that there is a little bit of separation between the outer and inner pieces, like this:

Original surface:



Inset with "Grooved" option enabled, with a small groove width:



- Michael