[Koha] Optional authority control in MARC21 700 field

[Rubén Fernández Asensio]

Hello all,
In my library, we've decided to do authority control for added entries 
only for personal names meeting certain requirements. (We're probably in 
violation of all cataloguing rules, but we just haven't got the time and 
means to control all secondary authors, and yet don't want to fully 
renounce authority control; there must be a middle ground.)
However, there's no way to activate the authority thesaurus only 
optionally and allow free text typing instead. I've tried adding a 
second 700 field with different requirements but it's not added. A new 
700a subfield simply replaces the previous one.
Is there a technical way around this? Or else, a better way of achieving 
this, and allowing one single record to have different 700 fields with 
authority control and without it? What would you do?

Rubén