filter by existing dynamic cck field values

Hello,

I wonder if it is possible to add an exposed filter in views, based on the present values in a particular cck field. (this field consists of city names)

As this cck field can have an undefined number of values (its values are imported by "node import"), it is impossible to manually set the "allowed values". It is a textfield. Each time a new node is created/imported, a city name can be added. I would like to filter the view based on the different city names that have been entered so far.

How could this be done?
Many thanks