With some black magic, everything’s possible 
It’s not obvious though, and not a recommended solution. This takes a button, extracts its code and replaces a piece of that code with the expression. Then creates an Action object out of that new adjusted code and sets it on another button. Please mind that the code uses slightly different notation than Coda formulas, e.g. you cannot reference rows and columns like you’d do in Coda formula language. For literal numbers it works though.