Setting multiple receipents (external mail adress) of mail step

I have a workflow which is Purchase order Recejted.

• I copied it and trying to send mail to multiple external mail adresses.
• I created an element in workflow container. SOXNA-FULLNAME (Multiline).
• I created the mail step. But I want to write a method to get the receipent mail adresses and pass it to this element and again pass element to mail step.

But I'm not sure where to write the method, I mean is it should be a step including a task etc.

Thanks for any help