create USB installer with patched kernel

[0x70b1 at web.de]

Hi guys, 

basically, what I want to achieve is making a USB installer of DragonFlyBSD 5.2.2 with a patched kernel.

I tried the following: 

1. download the official installer image (dfly-x86_64-5.2.2.img)
2. mount the image and copy the content of its second partition (the base system)
3. cd into the extracted base system and fetch the source tree 
4. apply the patches 
5. now I thought I could just rebuild and install the kernel and create an image again afterwards which I could then make an installer USB from. that didn't work because I couldn't even make nativekernel, buildkernel, buildworld etc. 

I doubt this was the right way to do it.. 

how can I create a DragonFlyBSD USB installer with a patched kernel?

thanks in advance. 

best regards, 
Tobi