So you have a CSV file with the post code, address parts and the possible house numbers for that postcode?
I don’t think you can do it as a dropdown directly, even it’s possible to make a one of the String fields act like a dropdown by adding a comma separate list of values into the Values field, I don’t think it’s possible to populate all possible values into it like that to then function as a dropdown.
One way you might consider is triggering an automation command to query the address then present the options in an Ask Question popup, it wouldn’t be ideal for a long list of numbers, but I don’t know the data you have. That would show a button for each number.
I am curious where did you get the data from? We usually use Google Maps API, which does not give house number level, but it generally free for this use case. Most other postcode data providers charge pay per use of of like £0.02 per lookup, but they do give live data.
If there is a source for getting “open source” or similar postcode data, I would be interested to know where.