These are the steps suggested by Apple for installing Mac OS X Server on hardware that is not supported by the Mac OS X Server media. The conditions for this procedure is that the version of Mac OS X Server be the same as the version of Mac OS X that shipped with the hardware. That is to say if you are installing Server 10.4.x, the original OS for the system must be some version of 10.4 (this is known to also work for 10.1, 10.2 and 10.3 systems).
(1) Perform a clean install (OS only) from the original media that came with your system.
(2) Once the installation is complete, insert the Mac OS X Server media. Navigate to the MacOSXServerInstall.mpkg (which should be located at /MacOSXServerInstall/MacOSXServerInstall.mpkg on the media) and double click on the package. Run the installation.
(3) After restarting the system, complete the Mac OS X Server Setup Assistant.
(4) Then run software update to bring the system up to the latest version of Mac OS X Server.
At this point both the version of Mac OS X installed at the beginning and the version of Mac OS X Server should be the same.