How can I add 'County' to the customer record area on Shopify?

Hi, I’m fairly new to Shopify but have found things relatively straight-forward so far. Until now. Having customised our checkout to suit our preferred UK format - such as including ‘County’, we now need to customise our Customer record area to match. This is so we are able to cleanly upload legacy customer data using the same format and so all our data follows the same format (Building/Company, Address, City, County, Post Code Country). So far however, I can find no way of customising the address fields in the Customer record area. This means things are fine when a customer enters their details at checkout as they will use the new checkout format that we want. However, when I manually add a new customer in the Customer area, I am not able to use the same fields, being stuck with Appt/Suite etc and no option for County (or ‘Province’). Any ideas out there? Thanks! Tom