Hello @Stan Verdiesen and @Anonymous
Welcome to the Q & A, and sorry for the late reply,
in answer to this issue, please be aware that The Remote Desktop Connection Manager (RDCM) program does not have an import feature for .rdp files, unfortunately, but you can manually copy the settings from the .rdp file and paste them into RDCM.
Alternatively, you can create a custom template in RDCM that matches the settings in your .rdp file to save time in creating multiple connections with the same settings.
If you found this helpful or have further questions, please be sure to let me know.