Pic shows a pat with a 3d sketched spline.  I want to make a cut through that profile and I'll need (I think) a control curve.  So I'd like to create another 3d sketch offset from this one.  I tried Offset On Surface but can't pick the 3d sketch.  I tried Sketch Tools/Offset Entities but that is greyed out.

I've seen this done on another previous users part, but can't figure out how to make it work. Suggestions?