Cant add my initial mailbox (same as my MS username) to Outlook(PC)

Hello Community 🙂 

Recently I got the basic Office 365 product and I installed it on PC. 
I setup Outlook and logged into with my Microsoft username (name.name@gmx.at)

Now i tried to get the initial gmx mailbox which is basically the same (name.name@gmx.at) as IMAP but it says "this account is already used". 
The window which lets you decide between outlook.com|O365|Imap|Pop3 does not open since it says it's already used. 

Is there any workaround?

Best regards 

Gabriel