It's sometimes possible to install the module after market, this can be done with the Mac Mini and Powermacs as far as I know. Not sure about Powerbooks.
Even if it is possible, it will probably be a lot more expensive than buying a USB adapter. Also consider the risks of fitting and labour charges if you pay someone to do it. The D-Link adapter is really very neat and small, see if you can have a look at one in a shop near you.