I just did a transfer in FRA (PDX-FRA-CDG) and went to the welcome lounge. Traveling in C there was no problem re-entering security to catch my outbound flight. The welcome lounge is very nice, great showers and a good food selection, I think there may have been 4 other people when I was there and the staff was excellent. They even offered to print my boarding pass for my connecting flight, so they know people are using the lounge who have connections, not just terminating flights in FRA.
I jotted down these directions when I was making the trek:
Welcome lounge out door B2 of baggage claim in B Hall, exit customs and turn right and it’s about 10 yards straight ahead. By airport exit door B6.