Hi,
Check it out...
http://www.5-axis.com/
I found this site recently. Thought it might be worth checking out, but I suspect mucho denaro
Also, I'm not sure if there is a system out there that allows you to supply your own spindle. Looks like this one is built into the design of the unit, as most probably are.
I don't even have a 3 axis yet and I'm already planning my upgrade to a 5 axis!
Carlo