Mirroring multiple source databases to a single destination in SQL Server 2008
Mirroring doesn't work like that. You can't bring multiple databases into a single database. You need to look to Replication, specifically Merge Replication as a solution for this. It can be set up across domains and it's specifically designed to take multiple different databases and bring them back to a single location. Mirroring is for creating a copy, a mirror, of a database. If you have more than one database, you'll have to maintain more than one mirror.