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 (not …). 

Materials that aren’t a preference (introductions, notes, citation rules etc) are best placed under regular headings, where it stays in Styleworks and is left out of PerfectIt exports.