How can I introduce WiFi into my wired 3Port ADSL2 router?
All the advice given above is good, but I would go with Mike's third suggestion, replace the router! It's simple, relatively inexpensive and the most hassle-free way of achieving what you want.
You would need a wireless router for making your ADSL2 connection wireless. its a pretty straightforward and easy setup.
I did the same for my clear internet connection at home. we got a Router, but needed a wireless internet connection. So we got a Belkin play Wireless router. Assigned access password to belkin router and connected the Clear router to one of its port. and it works absolutely fine.
Things are not as tough as it may seem, though, it would cost you a bit :)
Hook up the ADSL router to your main computer (assuming it has wifi capabilities) and use it to broadcast the wifi signal.
The easiest method is to by either a Wireless Router or Access Point and connect it to your existing Router via Ethernet.
With an Access Point the setup is relatively straight forward. But hey are usually a bit more expensive than Routers.
For a Router you have two options: Connect it by setting the WAN Port to extend your existing network (act as an Ethernet Port) or use one of the other Ethernet Ports on the device. The downside of using one of the LAN Ports is that a lot of the features (like the Firewall) won't apply since they are only looking at traffic going through the WAN Port.
Of course there is the third option of replacing your existing ADSL Router with an Wireless Router. In that case you will have to take another look at your current setup. If your current Router connects directly to the telephone line it has a built-in ADSL Modem which you will have to take into account when buying a replacement device.
The benefit of the third method (replacing) is that you prevent/minimize some problems like multiple DHCP Servers on your network or the need to reconfigure multiple devices.
Doesn't ADSL provide internet through a phone line? I am afraid that unless your router has an ethernet port and it has wireless connectivity compatibility you will not be able to. I would say your best option is to get a different router. Just my opinion.