termination field recommended for rehire can we add a new value to the list?

Summary

termination field recommended for rehire can we add a new value to the list?

Content

Hello,

In Termination screen  ,responsive UI, we have a field called "recommended for rehire" with list Yes/No/Not specified. we would like to add values to the list .
1) what is the list related to this field ? common lookup/value set etc?
2) will it not affect/damage the logic of this field if we add a new value to it ? 

see attached file.

Thanks a lot,
Revital