There are several ways you can add a return outside of your return timeframe.
One of the ways is using the Advanced Rules. You can follow our article here to set up an extended or different return timeframe.
Another possibility is to temporarily increase the return timeframe, submit the return through the portal and then set the value back to your default one. Kind of a hacky way but it will do the trick!
Go to Configuration -> Returns.
Change the Return timeframe.
Process your return.
Re-set the value to the default (correct) one.
Bonus: Staff members can also use the returns portal as well. It is three clicks, and then the return is in the system. To make it easier for staff, you can directly start a return with an order number and email in the link. To set this up, see the following integration guide.
If you have set a return timeframe and your staff return exceeds this timeframe, you need to manually extend the timeframe for the return and then reset it.