Of course it is possible to enter external name servers. Please follow the subsequent steps:
Log in to the customer area at www.inwx.com
Navigate to the Domain List
Click the little gear icon
(Action) at the end of the line of the respective domain name
In the popup menu choose External Nameserver
Choose a Nameserver-Set or enter the Nameservers manually
Confirm the changes by pressing the Save button
An update will be started to send the information to the registry
Pro Tip: You can check the outcome of the update in the Domain Log