Hi @Thanos,
Welcome to Algorand!
I believe you should be able to follow the last steps in the post How to recover my wallet with 24 word mnemonic - #6 by fabrice.
For the sake of completeness:
To recover the normal 25-word mnemonic from the hexadecimal secret key (e.g., 0d9cbee592b839f58814b84a77122d3d5939e745c328de99851a617b82e02598), you may run in the terminal:
$ echo -n 0d9cbee592b839f58814b84a77122d3d5939e745c328de99851a617b82e02598 | xxd -r -p > key
$ algokey export -f key
Private key mnemonic: like whip index matrix inflict diary picture retreat true endorse notable need increase keep boss churn design biology minute render agent join scorpion above roast
Public key: ZDGCBU2U2IEQWLXKP7G77XZWOXNYON4DQXZKEDYLUBJ4XR2UYNMF74TIP4
Disclaimer: Typing passphrases and private keys on a computer may open you to attacks that are not possible on the Coinomi hardware. Algorand makes no representations regarding the functioning or security of third party wallets, exchanges, or tools mentioned in the answer above, and disclaims any liability therefor. You are fully responsible for the security and the availability of your keys.