What you describe is a classic example of In-Application Programming.

For it to work, you must have designed the necessary support into your system (both hardware & software).
You will need:

The ability to send the code to the 320

Support (both hardware & software) in the 320 part of the system to receive the code and verify that it is complete & intact

Support (both hardware & software) in your system to transfer the received code from the 320 into the 68hc908's code memory and verify that it is complete & intact

Support (both hardware & software) in your system to start the 68hc908 running its new code

and last, but by no means least

Some way to recover if this all goes horribly wrong! eg, a power failure part-way through programming the 68hc908...

Unless all of this is already present and thoroughly tested in your system, you are doomed.