Do you want o move a mask path point which has an expression applied to it? If so:
If the expression is applied to a null object that controls the point, you can modify the expression by adding "value +" in the beginning. That way the original value will be added to the expression. You may want to start the values at 0.
Or you can use a slider control or point control and add it to the expression (pick-whip the controller value and add a "+" between it and the expression).
If the path is fully built with an expression (rather than via nulls), you won't be able to do this.