This is actually a pure MySQL question which should be titled "How to Alter a MySQL column". There is nothing different or special if the database is used for a Joomla site.
So simply to have this part of the question answered, a SQL statement like below would do it:
ALTER TABLE `YOUR_DATABASE`.`#__users`
CHANGE COLUMN `id` `id` VARCHAR(50) NOT NULL ;
On another note, of course there is nothing in Joomla that provides the option to alter its database schemas.
Also this is not recommended. Altering the db schemas of another application that you don't control and you depend on its updates etc doesn't make real sense. Future updates may override these changes.
Also I haven't review the user creation code, but it's very likely the ids to depend solely on the MySQL auto-increment function. If you disable this, you may open the door to issues and malfunction.
To be honest from your question I haven't understand what you are trying to do and the reasons why you want to change the
But since the other application is currently being built by you, why won't you adjust your application's id field?