You setup Sprint integration from the Google voice settings I think you want option #1. It will replace your Google number with your Sprint number then you can forward to your Verizon phone at no cost. You will loose your current Google number or you can start a new Google voice account to keep your old number but both accounts can not forward to the same cell phone.
Another neat trick is to use an OBI device on your broadband internet at home and plug any old landline phone into it to use as an VOIP phone at no cost or minutes using your cell phone number & Google voice. obihai.com/googlevoice.html