Adjusting the source of a Cube will only work if you have a table with an identical structure built into your data warehouse. If this doesn't exist already, then you would need to build out that table prior to adjusting anything about your cube.
That being said, if you're changing data sources from one version of NAV to another, it is possible to modify the project to work and still maintain any previous customization's made. This scenario also wouldn't require you to build out any new tables into your Data Warehouse, as it would use the existing tables but point to a different source.
If you're wanting to add the additional data source into your Data Warehouse and Cubes, but still keep all of the other tables and cubes pointing to your existing NAV database, then your project would need to be modified.
As you can see, there are a number of options depending on what it is you're looking to accomplish. I would recommend contacting your Business Intelligence consultant with Jet Reports, and setting up a Scoping call to discuss the details.
Please let me know if you have any questions.
two different databases