Question: How can I auto-increment alphanumeric characters in SalesLogix?

Answer: To do this you have to go character by character -

In SQL:
SELECT CHAR(ASCII('A') + 1) -- This will return B

In C#:
char src = 'B'; // 
Notice you don't use a String, but a character (thus the single quotes used as delimiter)
int val = Convert.ToInt32(src);
val++;
char res = Convert.ToChar(val); //res will be equal to C