[Replace in String] question

I am trying to implement a following simple transformation: NVL(cost_center_code, 0)
In the Calculator it only allows for NVL(A,B) where BOTH A and B are fields. What if I need to replace A with a constant (not value of other field)?

Also the final transformation is a bit more complex --NVL(cost_center_code,LPAD(traveler_cost_center,3,'0')). I do not see 'padding' function there. What is the way to implement that?

Thank You!