But I already have content on my xenForo forum and would only like to import the phpBB content into a new forum within my existing xenForo forum. The directions don't seem to accomodate for that. It seems to import the phpBB forum and it then becomes the whole xenForo forum.
The importer can be used to "merge" multiple forums together, just don't select the option to maintain IDs (as they'll already be in use). It will create new forums from your phpBB and import the content into those.
I would take a backup before you import though. (And both forums should not be accessible to users when the import is happening.)