That is perfectly possible.
But the default ASM2 fractionation model in WEST does not use those parameters: you will need to modify the model first, in order for the same parameters (e.g. i_P_BM) to be available both for the influent and ASU blocks.
I guess, you could add a variable ("V", algebraic state var) "organic P", computed on the basis of those fractions.
Then, yes, you can create a set of top-level parameters linked to the influent and to the ASU models.