Regular Keys (rekeyed accounts) can be imported into XUMM so you can sign with rekeyed Read Only accounts.
If you want to add an account to XUMM that uses a Regular Key (so the account has been rekeyed) or even has the Master Key disabled, you can import the primary account (the rekeyed account) in XUMM as a read only account.
Then import the Regular Key account as a read/write account, by importing the Secret Numbers / Mnemonic / Family Seed.
XUMM will automatically detect the regular key and the account(s) pointing to the regular key account.
If you want to learn more about Regular Keys and what they are / how they work, here's the XRPL Labs founder Wietse Wind explaining them: