I think that converting 5e monsters to FBL can work - but only up to a certain CR. The power scale of high-level D&D is super-heroic, and FBL cannot model PCs of that scale. I think the upper limit of FBL is around CR 13. That means creatures of the ball park: Roc, Vampire, Storm Giant, Beholder.
Monsters above that scale will result in FBL stats that are not fun. They just become a bag of Strength points to deplete and crazy overpowered attacks. Creatures like: Pit Fiends, Kraken, Ancient Red Dragon, Lich,