It only has numbers by default because that is the sequence number of the component.
The only way I see to use letters would be to write up a function in Java that takes the sequence number and looks it up in a table to see what the alpha version is. I say look it up because many (most?) people who use alpha skip I and O. If you don't skip them, then you could use the integer(index/26) and Mod(index, 26)+1 to look them up in the ASCII table. You could also break it up that way using 24 as the number of letters and look them up in a table that excluded I and O. That would give you over 500 index values before it fails, because 3 letters would be needed).
If you don't have the Java background needed, CeDeL Solutions (www.cedelsolutions.com/) does a good job on custom Java (They have done some minor functions for us and some major functions). Their web site looks a bit dated but I see that they list "Special automatic naming conventions" in their previous projects for customers.
We have had them in to train some of our people in how to use Java with Schematics.