We don’t currently have the ability to assign an upper level point to be a foundation node or spring, so I like to model part of the existing structure using lateral walls (or columns that are very stiff) to simulate the restraint from that structure. Then I make sure there are some lateral beams or horizontal braces connecting the newstructure to these walls.
If the new structure is only intended to lean on the existing structure in one direction, then the linking members can be pinned in the minor axis, or the existing structure walls can be in one direction only.
Here’s an example:
The only other tool is the Criteria - Ground level setting in Ram Frame, where the ground level diaphragm is laterally restrained in both plan directions.