Sure, you just need to do a bit of hacking to move the external module into the kernel source tree, tweak the Makefiles/Kconfig a bit so that the code is built-in, and then build your kernel image. For example, let's say you move the module source into drivers/blah. Then you should add a line to then end ofdrivers/Makefile like
obj-y += blah/ and you should make sure that drivers/blah/Makefile is set up to build your module in, with something like
obj-y += mymodule.o mymodule-objs := src.c other.c and so on, where your Makefile is set up however it needs to be to build the particular module you're working on.