For the implicit structural analysis used by Creo you cannot define a force as a function of an analysis dependent result/measure, only time. However, you can make an independent parameter have control over the value of the force in the force definition. The independent variable (parameter) is stepped using a sensitivity study, or optimized as a variable in optimization.
Actually time is just an independent variable for CREO, it is not really time, so there is no reason not use use it for X instead. This is because CREO is implicit and does not have dynamic effects. Try it that way and you should get the same result as my example above.