I think it is quite possible to implement a CAN using the UDBs in PSoC3. What needs to be evaluated is, do we have enough macrocells to implement the complete state machine. I am guessing that the resource available with 24 UDBs it should be good enough.
Just to add to the previous information. There is a datapath configuration tool that is installed with PSoC Creator when a full installation is done. This tool can be used to create custom components.