You may also do this from the menu at a later time, if you'd prefer.
PSE can import your existing bookmarks, though you'll just get the title and URL (that's all there is with old-school bookmarks). You can edit them later to add text descriptions as desired/needed.
Your in-browser bookmarks will not altered or affected in any way by this process.
Before importing your bookmarks into PSE, you must first export the bookmarks from your browser. How you do this varies depending on the browser.