I am not sure how FG Joomla works, but seems like it should be providing a csv or sql file of some sort. Why not just open that up and delete the old posts that way before re-importing into wordpress? Maybe it has an option to remember you last import and not over write the same posts created by the previous import?
Or if that doesn’t work, you could Un-publish all the posts that are in your current wordpress install (set as draft), and then do the import, all you have to do then is delete all the published posts in wordpress that older than the date you needed. Once you’ve deleted all the “older” posts, just set all posts back to active.
You could likely just tag them as well and use the same idea.