Is it possible to import BTC from another wallet and then claim BTG? Or can you just claim BTG if the BTC were on Exodus already?
No.
Tip: For that, you could use Bither[1]. They let you import private-keys and have a tool to claim BTG[2].
[1]
https://bither.net/[2]
http://docs.bitpie.com/en/latest/btcbtg/