How to transmit the signal of N switches with a CAN network
CAN works by sending frames.  Data frames can contain up to 8 bytes of arbitrary data, and have either a 11 bit or 29 bit ID.

In a very simple system, you could assign one ID to indicate the state of each block of 64 switches.  One processor would then monitor 64 switches at a time, and transmit the associated CAN frame whenever any of the switch inputs changed.

You don't need to use all 8 data bytes in every frame.  If 64 inputs is too much to handle per processor, then you could just send 4 bytes in each frame, and therefore monitor 32 switches in a block.  ID 0 could be switches 0-31, ID 2 switches 32-63, etc.