-snip-
However, if I understand well what I have to do is :
-Choose the option "Create New Wallet"
-When the dialog box asks "What kind of wallet do you want to create ? " --> I have to choose "Import bitcoin addresses or Private keys"
-Enter the private key of the "Cascasius" coin in the dialog box and click on "Next"
-After that, my Electrum wallet (the new one then) should show exactly 1.BTC as balance and I will have the possibility to send the amount on the exchange platform of my choice
You can still create another wallet while another instance is open,
use the menu "
File->New/Restore" to go to the install wizard, that will enable you to create a new wallet.
Just make sure to remember the wallet's name so you wont mix them up with each other if you forgot about the first one.