I have combined several percussion instruments into one part. Now when I want to add another lick for one of the instruments I find I am trapped in a section that is already assigned to one of the others. Can I “Uncombine” them for editing purposes?
I think you are referring to a 5-line staff?
If so, you can change the staff presentation type of Percussion Kits to see instruments individually:
Layout Options > Players > Percussion > select Single-line Instruments for the relevant kit.