In the CIW, if you type first:
Then you can enter SKILL++ code. You can enter resume() to return to the SKILL toplevel.
Or if the source is in a file, give it a suffix ".ils". Please read the chapters on SKILL++ in the SKILL Language User Guide - it's covered there - and in particular talks about calling SKILL from SKILL++ and vice versa, and how variables are shared between the two environments.