I "restored" a wallet using mycelium key. Then hit "add coins" in coinomi, and added BCH. Then hit "recieve" to generate a BCH address, and then sent BCH from armory to coinomi. It never gave me a new key, so i assumed it was tied to the iriginal key from mycelium.
A new phrase ? So you have one phrase from coinomi? use it! If you dont have a phrase from coinomi, do you have the SAME address this time, for BCH, when you generate the wallet from your mycelium seed, you don´t. If you have a new address and no phrase not much to do.
Coinomi might require you to generate the phrase your self, like with the priv key, not 100% sure thou!