I have an existing server running Cenos 6.x with Xen 4.x and a dozen or so virtual machines. I'm building out an OpenStack environment and was wondering if is possible to install OpenStack compute on top of the existing server and let it join the cluster, or if I need to build out a completely new server and transfer the the existing virtual machines over to it.
Unfortunately, there is no way to migrate running instances into an OpenStack cloud. The best you can do is stop the instances and import the virtual machine images into OpenStack.
You don't necessarily need to build out a completely new server, you may be able to simply install the OpenStack packages and make a few changes to your configuration. However, it may be simpler to rebuild the server.