At the present time, not all tcz extensions are in the squashfs format (it is a little different if they are in different formats.) In the future, they should all be in squashfs format.
To make a new extension, you need the squashfs-tools-4.0 extension installed.
This is how you can do it for those in the squashfs format.
mkdir -p /tmp/mnt-ext/dir2mount
Copy the extension to mnt-ext.
cd /tmp/mnt-ext
mount -o loop extension dir2mount
You can now copy and paste the contents of the extension from dir2mount to a new directory.
You can mount others in the same way and also copy and paste their contents, combining them.
In this example, the new extension will be made in /home/new-ext
mkdir -p /home/new-ext
Paste the contents from the original extensions to new-ext.
cd /home
mksquashfs new-ext/ nameofnewextension
Be aware, combining certain extensions, and using them this way, may cause malfunctions. Most should work OK. It would be helpful if anyone doing this, shares what they learn in this forum.
If you use these combined extensions, removing the original, and install additional programs, you may install dependencies which are already in the combined extensions. You will need to check them manually and delete them.