I haven’t converted Frostgrave barbarians into orcs with head swaps, but I have used Frostgrave barbarian arms (and gnoll and cultist arms) on the Oathmark orc bodies to get orcs with 2-handed weapons. You can see them here:
https://leadadventureforum.com/index.php?topic=132213.60. Pics of them painted are later in the same thread.
Putting orc heads on barbarian bodies might actually be easier than what I did (didn’t consider that at the time). One thing about that conversion, the Frostgrave barbarians and Oathmark orcs use a different joint to attach the heads. Frostgrave barbarians have a ‘neck’ that fits into a depression on the body, and the orcs have a depression on the back of each head, which mates with a lump on the shoulders. That is not hard to overcome (I find gluing a neck stump made of sprue off cut into the orc head is easy, then trim it to fit the spot where it will go), but it does add some steps.
Post some pics if you decide to give barbarian orcs a spin, I would like to see how that works!