This method will absolutely not work!
Opk is a squashfs filesystem. By renaming it to .zip you most likely tricked your extraction program to see a known file extension. It has however detected that it is not a .zip, but .squashfs and correctly extracted it. I have no idea what sort of a program you are using, however the dedicated tools for squashfs image handling can be found in "squashfs-tools" package, which should be part of every distro's package repository.
The following tools are used:
unsquashfs file.opk - to extract an opk (or just simply mount it)
mksquashfs directory file.opk - to create an opk with the contents from directory
To modify parameters of the opk such as the name, icon or executable, modify the *.desktop file present in the opk. More information can be found here