I assume your router is wireless, as you are connecting with the laptop. The answer is yes, you can buy a PCI wireless card cheap enough, but this would mean that you no longer have a "Hard Wired" connection to access your router.
Whilst it's possible, I would not advise it...It can cause problems if you wish to access the routers admin settings. Not impossible, just not something I would do.
Make sure that the card you choose is compatible with your operating system