Simply plugging in standalone WAP with the DHCP off and "bridged" mode on (if they have that option) will do the trick, but if it is for an untrusted network - I take it this is a coffee-shop type wifi setup - then you'll probably want to think about separating the wireless network from your business network via a firewall. Do you have a separate firewall already? If not you might want to invest in one, they can be reasonably cheap for a basic one. Then you can plug in the WAP and firewall to your existing ADSL router, then the rest of your network behind the firewall (on a different subnet).
There may be a way to do it using your existing router without getting a separate firewall, but it's hard to say without knowing what you have. There may be access points that offer separated networks and forwarding only to the default gateway (your ADSL router) which would achieve the same thing, I'm not sure.
Note that this will still offer no "traffic management" to stop people hogging your bandwidth or blowing your monthly cap, though again depending on your existing router you may have some rudimentary filtering/throttling controls.
One more thing - make sure you change your router password from the default (probably "admin") to something more secure :)