How to include packages that arent in the repo when building an image?

Based on this wiki We can build openwrt images like this:

make image PROFILE=XXX PACKAGES="pkg1 pkg2 pkg3 -pkg4 -pkg5 -pkg6" FILES=files/

Note that the list of PACKAGES must be in the openwrt repo.
But I have my own packages that aren't in the repository, they're ipk files that I compiled on my own. How do I get these packages installed during the build ?

I haven't used imagegenerator as compiling the whole firmware is easy enough, but two approaches come into mind:

Wiki is your friend. Both answers are directly from the image generator wiki page.