I’m sorry there is no UI to import a key on the device at the moment. You have to use the command line. But it is using standard tools from the GnuPG project. The only drawback at the moment is the ancient version of GnuPG that is in use.
On your desktop, you need to export the key (public and private) that corresponds to your email (see for instance https://unix.stackexchange.com/questions/481939/how-to-export-a-gpg-private-key-and-public-key-to-a-file). Copy the file to the device.
On the device, import the key (https://unix.stackexchange.com/questions/184947/how-to-import-secret-gpg-key-copied-from-one-machine-to-another).
Then, you should be able to see your key listed in the crypto section in your email account setting page.