I'm currently moving my code from SQL Anywhere to MySQL. The cleanest method I found with MySQL is to use a variable to do the math and then assign the variable to an alias to show the results.

@val1 =  SUM(currentbalance), 
@val2 =  SUM(principal),
@val1+@val2 AS 'Total',
// to show the values at columns
@val1 AS 'current',
@val2 AS 'prin',