Unfortunately, the configuration file will be included only in the kernel image only if its compilation indicated (most of them do not). You can see /proc/config.gzif you are lucky, he will be there.
If you have the kernel source, take a look at /arch/arm/configs- most of the Android kernel that I saw will have a default configuration for your processor, and you can start from there.
You can compile an external module for the Android kernel in the same way that you compile a regular Linux module. Just make sure that you are using the kernel tree that you downloaded as the kernel headers. It may or may not work depending on how close the kernel is loaded into the currently running kernel.