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.
For example:
m/0/0 ->
m/0/1 ->
m/0/2 ->
m/0/3 ->
m/0/4 ->
m/0/5 ->
m/0/7 ->
m/1/0 ->
m/1/1 ->
...
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.
custom m/0/0 isnt matching any of my known addresses it seems. did you do this manually and guess to find the correct one? is there a way to search in mycelium for the derivation path? – Patoshi パトシ – 2017-08-02T18:31:29.390
1I had to manually iterate through the m/0/0 -> m/0/7 etc in order to find the address with the unspent output. Starting with m/0/0 you should be able to find address matches for that particular HD account. Double check to make sure your private extended key is right.
You can also select the account from the accounts tab, hit the "Sign Message" menu option to view all your public addresses associated with the account. – Jazzer – 2017-08-02T18:37:01.353
i have over 40 public keys. does it always start at m/0/0 and increments up from there by m/0/1 ? – Patoshi パトシ – 2017-08-02T18:45:21.903
1The account I used this tactic with had 12 keys, and I was able to find each used address starting with m/0/0, incrementing to m/0/7, at which point I had to find the rest starting at m/1/0 and my last key was found at m/1/3. I'm not certain but I would guess yours starts at m/0/0 and your 40th key would be at m/4/7 if the pattern keeps repeating. – Jazzer – 2017-08-02T18:49:20.283
For me the custom derivation key was m/0/14 then it went from m/1/0 to m/1/40 --- You will need to manually iterate through it to find matching public keys with another window open listing all the public address linked to the xpriv key. It took a good 1hr to do this. – Patoshi パトシ – 2017-08-03T20:26:22.177