keeping database columns consistent with documentation
If the only reason not to add columns is "but that would make the database design, in documentation, inconsistent with the columns present in the database" then you need to look at your procedures.   

If there is a business reason to add the fields and your systems support it then you need to have your documentation updated at the same time as the table. This should not be much more than adding a description fo the field to the documentation, perhaps having a place in documentation just for these user added fields.

Note that the business wants the system and the documentation is only a tool to help and not the end result.