I think the syntax allows it now. But it's not easy: use the syntax: test_condition ? result_if_true : result_if_false which is similar to Excel's syntax if(test_condition, result_if_true, result_if_false).

For​ example:

avg( (q1>=20 and q1<=29) ? 24.5 : (q1>=30 and q1<=39) ? 34.5 : (q1>=40 and q1<=49) ? 44.5 : null )