That information does not have to be numerical. There are two precinct fields, precinct_name and precinct_code. The code field is numerical, but it is the only voter ID field that is limited to numerical input. 

We have a lot of resources to help you import people into your nation. There is a specific page explaining how to import a custom voter file that you may find helpful. 

Be aware that auto-districting will supersede political districts added via import. If you would like to maintain complete control over political districts, you'll need to turn that feature off before importing your voter file.