forced clocking

[rbussell at mlsma.mlm.att.com]

Hi,
  Does anyone know whether it is possible to take a microcontroller type IC of the 
sort that you just need to connect a crystal to for a clock.. i.e it doesn't have a 
clock in pin.. just two across which a crystal or ceramic resonator is connected, 
and actually give it a clock input so that it's operating frequency could be adjusted?
(sorry about that convoluted sentence :-) )

Any comments appreciated,


                       Cheers,
                            Robin