The user can add other domain tables and columns while creating/editing report.
Cross domain Tables and Report Columns :
1) Add a report column from the table and if the table has joins from different domain, then those tables will be added to the report column list and you can add those report columns from that tables, below is the screenshot
2) In data source tab, user can see the join table details from different domain.
2) In data source tab, user can see the join details from different domain based on the report columns.