Modify stored procedure in MSSQL

I have a stored procedure written that updates some table fields in my database, what I need is the stored procedure modified so that if @lowestsupplierID > 0 then do what its currently doing but if @lowestsupplierID = 0 then run a different update equation. I can write the different equation, what I dont know how to do is get the @lowestsupplierID to carry over from a previous stored procedure into this stored procedure so I can adjust the different equation. You can either remote connect into the server to make the modification or I can email it to you. It should be a very simple project for someone who knows what their doing, a few minutes really.