How to create a custom asset placeholder from cad line work
This may not be possible. But once the custom asset is created, you can associate the custom asset to any Revit family component. You may need to adjust the origin and orientation of the Revit family to match the custom asset.
I do this with planting families that only contain model linework in plan (indicating the size of planting) and then associate the Revit family to the Enscape asset (trees and other planting). This way the building isn’t obscured in elevation in Revit, but appears fully rendered for presentations in Enscape.