How to add wifi

Hello,
I have adsl internet hookup. The internet modem & router are in one physical box. There is no method to remove the router. There are 2 desktop computers attached to the router. The phone line connects to the box using a proprietary(?) connector. There are 4 outputs (downstream ports). It has no conventional Wan port.

I need a wireless connection to use with a tablet.

I am willing to buy some hardware, but what?

Can I add a wireless router (in addition) to the wired router - plugging it into a downstream port - or?

Can I add "something" to one of the desktop computers so it could talk to the tablet? Expansion card?

One dealer suggested that an ASUS "Portable Wireless 4-in-1 Router" (WL-330gE) would do the job but he couldn't explain how to hook it up. No sale!

As you can gather I'm a wifi newbie. Is there a solution?

Thanks