I would check first if your old router has the wireless repeater function first before going down the custom firmware route.
Installing custom firmware
Some routers donít have repeater mode built-in, while others have the capacity but require you to install custom firmware to unlock this functionality. There is a risk that you could brick your router and, although most can be revived by sticking a pen into its reset button, it could invalidate the routerís warranty.
Because of this and a more convoluted process, only advanced users should consider this. The DD-WRT website provides custom firmware and specific instructions for some routers.
If all else fails, you may need to buy a wireless booster or new router. Wireless networks arenít always ideal.