The syntax is ".cmdtree c:\debuggers\cmdtree.txt" where cmdtree.txt is a text file describing all of the commands.

This was picked up on several popular debugging blogs, such as:
http://blogs.msdn.com/debuggingtoolbox/archive/2008/09/17/special-command-execute-commands-from-a-customized-user-interface-with-cmdtree.aspx
http://www.wintellect.com/CS/blogs/jrobbins/archive/2008/09/17/windbg-cmdtree-file-that-eases-some-sos-pain.aspx
http://blogs.msdn.com/tess/archive/2008/09/18/making-it-easier-to-debug-net-dumps-in-windbg-using-cmdtree.aspx

An example of this is provided here:

windbg ANSI Command Tree 1.0
title {"Commands"}
body
{"General"}
  {"OS Version"} {"vertarget"}
  {"Computer Name"} {"dS srv!SrvComputerName"}
  {"CPU Make/Model"} {"!cpuid"}
  {"Manufacturer"} {"!sysinfo machineid"}
  {"OS Kernel Version"} {"lmvm nt"}
  {"OS Kernel HAL"} {"lmvm hal"}