No built-in WiFi on a tablet? I've never heard of that. Unless may be it's a model that works over a GPRS mobile network in conjuction with a mobile phone.
One thing's for sure, you can't add wireless capability to it after it's left the factory as there's no suitable port to plug in a USB wireless adapter (requires Standard-A USB slot like you see on computers; tablets don't have any of those).
But I still find it hard to believe your tablet doesn't have built-in WiFi, unless it's broken.