NOTE: Personally, I don't recommend this.. it's potentially exposing your private key and could lead to loss of coins if you're not careful!
Use the "Add Account" -> "Advanced":
As an alternative, instead of creating a new account, you can just click SCAN from the main screen, scan your private key's QR-code, and sweep the funds to your existing Mycelium account.
Warning: don't expose your private key to online QR-generators!