Is it possible to setup call forwarding for external incoming calls on an internal phone if I am using a Freepbx system?
Just for external calls inbound, normal ext to ext calling will still work like normal.
You may want to explain your scenario or desired use case in more detail to get better suggestions.
Neither Call Forward nor Follow-Me are currently designed to work differently for internal vs. external calls. However you can clearly route a call designated to a DID to where ever you want it to go and all external calls should or can come in through an inbound route.
I suspect once more detail is provided, other suggestions can be made to accomplish your end goal.