Ok, thank you.

I was confused because on the bottom of the Electrum wallet Balance screen a button "Sent" is displayed.
A "Sent" button should indicate a full rather than a "Watch-only" wallet.

This should be changed in the next release.

A look into wallet details shows "watch-only". ok.

So can I turn this watch-only wallet subsequently (=now) into a full wallet?
Simply by importing seed or private key?
Or do I need a full new installation.

Later I want to toggle this wallet occasionally from watch-only to full and back.
Is this possible without fiddling with seed and/or private keys?