The body from extracted geometry is probably still a sheet body.
You cannot unite solid and sheet bodies.
Place the cursor on top of the body from extracted geometry, then wait for the quick-pick three dots to appear, then select and in the quick pick dialog scroll down to the bottom and note if it is a sheet or solid body.
When you sew sheets, if they are watertight within the tolerance and logically possible to convert into a solid, the result will become a solid body, else NX sews a sheet body.
Regards,
Tomas