Can I connect a CAN node with 250 kbit/s to a CAN bus running at 500 kbit/s?
Basic CAN protocol doesn't support nodes running at different bitrates:


  The speed of CAN may be different in different systems. However, in a given system the bitrate is uniform and fixed.


The newer version of the protocol called CAN FD, provides support for flexible data rate, as its name suggests. Legacy CAN hardware is supported on CAN FD networks, however, it still has to run at a given fixed bitrate.

I think your only option is to buy a router. Here is an example of a CAN router which supports 4 channels with independent bitrate configuration.