I think your best solution (especially if you need to do this more often) is to make use of metadata injection based on infrastructure files of the database. For example load a list of tables and fields per table in pentaho, and feed this in a metadata injection process where you have a table read and write function. In between, (or even in the read SQL) you can push a encryption step which encrypts the data if the field is present in a certain list (or even better store the list in a database).

This way you just have to write the metadata injection once, and fill the "to be scrambled" list once. and then you can run the process on the whole database at once, and asa often as you want.