Whether or not you can do this depends on a few things. The motherboard you have (if its hardware and BIOS will allow the use of the onboard graphics and a discrete graphics card at the same time), the make of the onboard chip and discrete card and your computer's operating system.
For example, my home server's motherboard (ASUS M3A78-EM) has built-in support for "Hybrid CrossFireX". This apparently (I've never used it) allows you to team the onboard graphics chip (Radeon HD3200) along with a discrete ATi graphics card. However, it does state that this can only be done using Vista and there is a restricted list of discrete graphics cards you can use.
I've never attempted to use it, but if you provide the details above maybe someone else could help you out more than me.