The only issue with this is that they will both communicate in the same area, which could cause interference and wireless disruption if on the same channel. If you can access the other AP and change the settings and channel settings, put them on a different channel.
Are you losing connection to your wireless now? Are you at LOW?
If it is at LOW, you may even be able to get away with using the same SSID and security settings, matching the existing wireless. This will allow your computer to move freely around the office without losing connection.
If your office uses a router w/wireless, I suggest using the same manufacturer. If it is a stand alone AP, use the exact same one.
On a side note, if you are able to access the wireless everywhere, but just have a LOW signal, do you notice performance issues or a total loss of signal where you are trying to access it? If you keep connected, I recommend not messing with it. Try moving the AP that exists closer to the other side of the building, maximizing its range throughout the office