I can't tell you the exact steps, but there are lots of good guides out there that will explain how to build a new kernel.
In order to make Bluetooth a module, when you configure the kernel (make menuconfig), find the Bluetooth options and press M for module. You'll probably want the Bluetooth module itself, the drivers for your specific Bluetooth device (or include them all if you don't know which chipset yours uses), and all of the protocols. Finish following the rest of the steps for building a kernel and you should have a kernel ready for modules as well as all of the modules installed. Reboot, plug it in, and cross your fingers.