It's possible with the right calculation. Did you really mean "across multiple fields"? Or did you really mean "across multiple records"?
Here's a hint to point you in a possible direction:
Char ( Code ( previousletter ) + 1 )
Char ( Code ( "A" ) - 1 + SerialNumberField )
Note that this limits you to a single letter sequence of characters. A more sophisticated approach would be needed to work with multiple characters.
Yes, i meant across multiple records. I will give that a try and get back to you.