Invoice Schedule

Hi Experts, 

I want to add one more line (Advance Payment) in invoice schedule, when Generate Periodic Milestones is pressed, other than billing schedule. 

The date of invoice has to come from a custom field which is present on the form below billing start date.

Is it possible ? How can i access the workflow or command which creates invoice schedule according to billing frequency? If I manually add a line and create periodic milestones, it deletes my custom entry.