FInd a hotel loyalty program in the middle that allow you to transfer in this way FB -> Hotel -> Avios

Check the ones you might already have , maybe they partner with both flying blue and your avios based FFP

Now as you can probably imagine the two folds conversion will be done under two not so great rates , usually between 1:2 and 1:10 , so unless you have very big amount of FB miles and have no skyteam flights or hotel stays scheduled in the foreseeable future , then i dont recommend to do it