I can't speak for the coinomi app importing an extended key, but if you need to extract the private keys from the extended key, you can use http://bip32.org/.
I haven't personally audited this site, but I have used it to browse my private keys associated with a Mycelium HD account. Once you have extracted your private extended key from Mycelium however, I'd recommend archiving that account and making a new one.
The keys can be seen by first entering in the entire extended private key, then choosing a custom derivation path. I've found my Mycelium private keys starting with the custom path m/0/0. From here, you can increment the last part of the path up to 7 before incrementing the middle part of the path.
Edit: I have successfully used the Coinomi app to sweep my unspent outputs from the Mycelium app. I first entered the private extended key into the appropriate box on bip32.org, and I browsed through the custom derivation paths until I found the address that had the unspent output, which also displays the private key. Entering that private key into the Coinomi app allowed me to sweep the balance of Bitcoin Cash that was in those unspent outputs.