Where are you trying to import your sub models from and are you thinking some type of batch import or one at a time? What does " can't import mean?? You can not find, get bug splat etc.?
If your sub models are components you could make them a collection and save and then they would be available from the component browser like other components ,or you could make a model folder, load the skp files in that then import 3 d models from there into the target file.?
I need more detailed  explanation of what you want to do and what can't means?