As far as I can tell, CruiseControl.Net has no support for creating farms - at least it doesn't work the way you describe. Interpretation of CCNet "farms", apparently, assumes that the projects are assigned manually to the machine, and this project will always be built on one computer.
If you want to dynamically choose which machine is actually assembling, you will need to create your own mechanism to select this computer and start it. There is likely to be a complex difficulty with this. For example, you will probably need to ensure that the same project is not created simultaneously on two different computers if the second commit occurs while the previous commit is still being processed.
If there is a shared access that all build machines can access, it may be possible to use the file system source control block or the ForceNet Build CCNet engine to run the build on the designated computer, but all build machines publish their output for this project in the same final place.