identity on case sensitive varchar field

hello,

i need to make my already case senstivite varchar(6) field the identity for my table.

I want that instead of autoincrement it generates new keys, it can use numbers and both lower/upper case letters. Currently im doing that programmatically but i wish to integrate this function in my database, so i do not have to check if a generated key already exists from source code.

ive read about NEWID() function but i dont know how/if it can be personalized for this purpose, it seems it just generates GUID formatted strings.