Advanced Search Fields – Custom Post Codes

Hello again 🙂 (ive been very annoying these few days)

I have a custom value for postal codes when importing. The value I have set is map_zip. When trying to add to the advanced search field, map_zip shows up but doesnt give me the awesome functions the inbuilt zip code search does. Is there a specific table i should be importing the zip codes to rather than my custom field to be able to utilise the inbuilt function?