Are you wanting to capture this from forms?
If so, you can add the field to the form and pick 'Select' as the field type. Then, you can edit the options of what someone can select for that field and the format of how that selected value is stored in SFDC.
If you are truly building a segmentation for that has segments for those values then you can use the trigger 'Segment Changes' and select the segmentation and add a constraint for the segment value. Within the flow you would then 'Change Data Value' for the correct field value that corresponds to that segment. To update everyone already in the database you would run a batch job using the same functionality except using a filter for the segmentation name for each of the segments.