If you use the Converter Plug-In from VirtualCenter, any conversion can be scheduled.
Problem is, if you choose the source as a "Virtual Infrastructure Virtual Machine" ... it needs to be powered off at the time the wizard is run. To choose the "Virtual Infrastructure Virtual Machine", a vm must be powered off, and that's pretty much it.
Alternately, you could use "physical machine" as the source. However, this would require the VM to be Windows based, but would let you generate a conversion.
Lastly, you could schedule a command line conversion using p2vtool .... but you'd need to search the forums and documentation on how to use it.