Hello and welcome to the community Marco,
If the RDX system can be mounted to the OS with the native tools ( in order to be presented as a regular volume) or presented as a file share – yes you can use it, but please keep in mind that there is no additional care for RDX systems. Since Veeam Endpoint Backup is completely free, please, test the described scenario and share the experience with the community.
P.S. I wonder if the RDX can be somehow managed by the existing Endpoint’s rotated media mechanism.