Afaik the Load As Group command is not exposed in the Revit API. One thing you could try:
- Load the file via background open
- Group all elements in that file
- Copy the group instance to your target file
All of these commands are exposed in the API and I’m pretty sure they are available as custom nodes in one package or other. If you had one Revit file that already contained all the groups you need, the process would be even easier.