Forgot to mention.
The only experience i had on building an andoid kernel is just compiling it.
Idk if it even considered as an "building" experience.
For rom building. Actually never done it before.

After reading a lot of docs. (Can't cite a reference. Sorry. I accidentally deleted my browser app data that contain a few site that has a lot of related information)
The answer to this question is "you can"
As long you got the kernel and device blob source code for the phone you're using.
But this would be a tedious task