exporting variable from User Defined Commands

I'm developing some gcode for my partner. Unfortunately I'm writing the code from a remote location and I have a question for which I cannot test to get my own answer.

If I execute a User Defined Command (M100-M199) is it possible to either assign a value to a gcode variable from the command or return a value and assign to a gcode variable as a result?

e.g. I want my gcode variable #50 to be set to a value determined by by user defined command either explicitly in the executed command or assigned from return value... something like #50 = M199

Possible? If not, I need to alter my program's logic and was hoping to find out now rather than have to wait to test in person.

Thanks all!

Please disregard my question; I can accomplish what I need by writing to a file and reading the variable with an oX call. Also, this should have been in the gcode forum, sorry.