Graphic timers are external - not something you can add to your existing PassPorter timer. You have the option of adding an external countdown timer to your signature. Try a website like
Graphical Event Ticker for your Signature, Message Boards, Blogs and Web Page or
MouseHouseMagic Disney Community - Home for some fun graphic timers. When you've created your timer, just copy the UBBcode option and paste it into your signature like you would any other text or graphic.