Installation without ISO and VHD

I would like to replace the existing Zentyal server. However, there is no chance image files to install. Do you have any solutions, just as in the case of Zentyal to put on my repo and installed using apt-get install the necessary components? Any help and advice is welcome.

Can you explain, why you cannot use image files? You would at least need to install the base system via image, befor you can use apt-get to install additional components.
Maybe an image on USB or the likes is an option?

Unfortunately, all I get on the system, a template installed Debian.

ah okay - so you want to install the template debian and then modify it to get an UCS - all via apt-get? Unfortunatly, there is no documented way to do this. I do not know if it is possible.