Mapping 2 form fields to one column

I have a lookup column that I use on a Nintex Mobile form. The list of values is fairly long, and occassionally the field personnel cannot find the value they are looking for,  I want to add an additional field - like an “Other” on a normal choice field.  This is not an option on a lookup, so I thought I would add a checkbox - “enter value” - which would disable and hide the lookup, and make available a free form entry field.

Both the lookup and the text field would be mapped to the same SharePoint column.  Is this possible?  I know I have done in the past, and it seemed like an issue, and I don’t recall ever figuring out the “trick” to make it work.

Thanks in advance!