Customer Exit Formula Variable using Replacement Path Formula Variable

Hi All

I have a requirement where in I have to create a Fomula variable (F1) using replacment path  and this variable contains the Key value of the characteristic it is based on.

Now, I want to this formual (F1) in anoter Formula Variable(F2) which is of the type Customer Exit, as I need to use the Value of F1 to do some back end logic in the customer exit which is not possible using Bex Formula's.

Now I understand that Customer exit variables are called only during I_STEP , but the value for my F1 gets filled post the I_STEPS. How can I use the vlaue for the F1 in the F2 customer exit?

Regards,

Manpreet