Is it possible to add a letter or some other character in front of an incrementing field?
It is not possible with an auto-incrementing field, exclusively.  You could set up another column inside each table with a single letter identifying the table, have an auto-incrementing column, and then make the key the combination of the two.  You can also set this up using a trigger.
However, this doesn't fully make sense because the letter really isn't needed within a single table.
I suspect that you are trying to solve a different problem, which is to have multiple "types" of an entity in different tables (say a table of contacts, with one for email, one for mail, one for telephone contact).  If so, another approach is to have a master table of everything with an auto-incrementing id, and then to use this id in the subtables, defined with a 1-1 relationship.
What I would like to do is have my primary key field (or another field as long as it accepts auto incrementing) auto increment with a letter in front of it. For example:
I would like to be able to make pk equal A1,A2,A3 in one table, and if i choose, B1, B2,B3 in another table. Is this possible with django? mysql? The field doesn't have to be the primary key field as long as it auto increments. Please let me know.