In Xumm it is possible to import an account in Full access mode or Read only mode.
In Full access mode, you can interact with your XRP Ledger. ie. if you would like to create a Trust Line or send XRP to another account.
In Read only mode you can follow the activity of an XRPL account but not sign transactions on the account.
If you have imported an account in Read only mode and would like to convert it into Full access mode, (so you can sign transactions), this article explains how to do this.
1) Launch Xumm then press the Settings button, choose Accounts then press the Edit button beside the account you would like to convert.
2) Tap the Access level option...
then choose Full access.
3) Press Yes, I'm sure.
4) Choose the account secret type for your account.
Note: Xumm is compatible with all industry standard account secrets. When you created your XRP Ledger account you would have received one of them. You need to have the correct account secret for your account.
From here, get your account secret then choose the appropriate path and follow the instructions to complete the import process.