I was just doing this myself--if you're working from an existing relative who matches someone in the main tree, once the compare screen has just an approved match, and all bad ones have been ruled out, when you re-open it, there will be an ADD button in front of the unmatched entries. (For example, if you have a Great grandfather in the world tree, and want to add a spouse or child, once the GG is matched correctly, the next time you click on the compare button for that person in the gedcom list you'll be able to add people.)
If you match or rule out the options for everyone in your whole tree, the ADD buttons will show up in front of all unmatched people, but even after that I found it easier to work from existing records to make sure I wasn't adding anything in the wrong place/keeping the connections accurate.