Is it possible to modify field types of mandatory fields?

Is it possible to modify field types of mandatory fields?

Hi,
Some fields such as address, city, country are mandatory fields and I can't modify their types since they are fixed to text type. Unfortunately, many of our sales guys make stupid mistakes when typing city names; therefore, I would like to switch these text fields to combo lists in order to avoid these mistakes. Is this possible to do? If it is not possible, I would like to know how I can create a new field value which can be seen by other modules. For instance, I would like to create a new city field which will be propagated all the way til sales modules.

Thanks.