With Maintenance Planner you can generate stack.xml from SAP Service Market Place (SMP), all you need is to assign your Solman User to S-User id.
Maintenance Planner is hosted on SAP Support Portal. The landscape data used for all planning activities is uploaded into the customer profile via Landscape Management Database (LMDB) of SAP Solution Manager.
How authentication works?
Authentication works based on the s-user being mapped to customer number so each customer will see only their own data.