Hello Community.
On GMX i have an Email Account with, not sure how to call that in english, “Sub” Emails.
It ́s Email “Adresses” added to the main account.
I was on Thunderbird before where it ́s quite easy to add the “Sub” Emails to the Main Account but want to give “EM-Client” a try now so i would like to know if it ́s possible to add these sub mails to the main account in any way?
I think you mean aliases. They will be delivered to the same GMX Inbox, but to send using them you need to add the alias to your account settings.
Go to Menu > Tools > Accounts > General tab and click on the Alias button in the User Information.
Once you have added the aliases, when you compose a new message, you can choose which address it will be sent from.
Hello,
you mean Mail alias?
With your eM client you can go to the tab: -> Accounts -> then “gmx” -> Alias -> and there add your further mail address (alias address).
Ah yeah aliases… Sry Guys… No native english 
But you put it in a nutshell
Thanks!