How to use variables from C-Code in a matlab function block?

I want to generate code from a simulink model with the embedded coder. The generated Code is used in another software project that already exists. In this software project some variables are already defined. In the simulink model, I want to use this variables in a matlab function block.

How can I tell simulink that the variable Phase_Current_U is already defined in the C-Code from the software project?

function adc1 = fcn 

adc1 = Phase_Current_U;