[Isa Alsup]

When you click to make a new expression, the expression box is populated with an expression that means “use current keyframes”

One easy way to do exactly what you describe is to add your expression to the current parameter value. Even if there are keyframes it works. Also if you edit the value or keyframe, you are editing it before the expression (it shows value after expression). Remember with 2 dimensional or 3 dimensional parameters arrays you need to use vector math or operate on each part separately.

I commonly will animate a parameter 0-1 or 0-100 then multiply it by a constant in the expression. In this way one animation could drive the fade for items with different transparency levels.

All the best