Not a direct answer, but workbooks appear to be (4.2) represented in the database as a collection -- for instance, a workbook called Test in a previously constructed folder called Slamdata is represented by a collection called
SlamData.Test\.slam.index
which typically has a single document describing the workbook
If you use a tool like Robo3T to visually manage your database, you can then do two very useful things that aren't in the UI as far as I can see:
1. Make a copy of the workbook by copying the collection. This is very useful because it's very easy to corrupt a complex workflow in the UI and have to start over. Now you can do interim saves!
2. View the workbook document as a JSON document. Then insert it in your target DB where you want. Haven't tried this one, but seems likely.