Formula variable based on customer exit variable

Hi expert

I have created a customer exit which calculated the last day of the current year: 31.12.YYYY.

This value is assigned to a BEx varialbe (customer exit type).

Now I would like to use this variable value for calculations. Therefore I thought to create a formula variable based on characteristic 0CALDAY. Under replacement path you can define the object, infoobject or variable. If you select variable I try to find my customer exit variable, but don't find it.

Is it possible to define a formula variable based on char CALDAY, replaced by a customer exit variable?