if you were in control of the router(meaning if you could change settings on it), you could purchase another airport[xpress] router and set it up as a wds. i copied this from the airport help viewer:
Extending the range of your AirPort network using a Wireless Distribution System
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You can set up multiple AirPort Extreme or AirPort Express base stations as a Wireless Distribution System (WDS) to extend the range of your AirPort wireless network. When you connect base stations wirelessly in a WDS, you set up the base stations as a main base station, a remote base station, or, if you are using AirPort 3.1 or later, a relay base station.
A main base station is connected to the Internet and shares its connection with remote and relay base stations. A remote base station shares the main base stations Internet connection. A relay base station shares the main base stations Internet connection and transfers the connection to other remote or relay base stations.
All three base station configurations (main, remote, and relay) can also share the main base stations Internet connection with client computers wirelessly, or with Ethernet if the client computers are connected to the base station by Ethernet.
When you set up base stations in a WDS, you need to know the AirPort ID of each base station. The AirPort ID, also known as the Media Access Controller (MAC) address, is printed on the label on the bottom of the AirPort Extreme base station next to the AirPort symbol, and on the power adapter side of the AirPort Express base station. To make it easier to set up a WDS, place all of the base stations on a table and plug them into a power supply.
If you are setting up AirPort Express in a WDS, use the AirPort Setup Assistant to set up a WDS.
To set up a main base station and remote base stations in a WDS:
Use the AirPort status menu in the menu bar to join the wireless network created by the base station you want to set up as the main base station.
Open AirPort Admin Utility, located in Applications/Utilities.
Select your base station and click Configure.
Enter the base station password if necessary.
Click WDS.
Select the Enable this base station as a WDS checkbox, and choose main base station from the pop-up menu. By default, the "Allow wireless clients on this base station" checkbox is selected. If you deselect the "Allow wireless clients on this base station" checkbox, and later want to change the settings on the base station, you must connect to the base station's LAN port with an Ethernet cable. You will not be able to connect to the base station wirelessly.
Click the Add (+) button to scan for base stations in range of your computer. Select the base stations you want to set up as remote base stations. Deselect the Auto configure as a WDS remote base station checkbox if you want to set up the remote base station manually. Click Rescan if a base station doesnt appear in the list. Click the Delete () button to remove base stations from the list.
Click Update to send the new settings to the base stations in the WDS.
As part of the automatic setup process, you can give each base station a name and password, and choose how the base stations receive IP addresses.
If you want to add additional remote or relay base stations to the WDS after setting up the main and remote base stations, use AirPort Admin Utility again.
Remote base stations need to be on the same channel as the main base station. Before setting up additional remote base stations, find the channel of the main base station in the AirPort pane of AirPort Admin Utility.
If you are using access control, you need to enter the access control list on all base stations in the WDS.