Can I paste my existing style sheet into Styleworks?

Yes. You can type or paste terms straight into a Preferences List; you don't have to add lines one at a time with the green plus button.

After pasting, scan the info bars at the right of each line. Green lines are known terms, ready to export; red lines are unknown terms, you can edit and add the search term with the reserved word (not …). 

Materials that aren’t a preference (introductions, notes, citation rules etc) are best placed under regular headings, but you can still place them anywhere, and they won't be exported to PerfectIt unless Styleworks detects them as a known term.