It is possible to add more systems to an existing system landscape. However, these systems will need to use fresh installations of the application. That is, it is not possible to move an existing application installation into a different system landscape.
The steps involved in adding a new system to an existing system landscape are the same as those described in section 5.4.5 - Setting up other Systems in the System Landscape.
It is worth emphasizing that the key block range allocated to the new system must not previously have been assigned to an existing system in the system landscape.