The yearly quantity of parts will determine if a post process or insert molding process would be better. What is your yearly volume expectations? Do you currently have tooling built? It would be tricky to modify exsisting tools. Does the "insert" need to be totally enclosed? Either way it can be done. It would just take a bit more engineering to totally bury the insert within the plastic. They do this with golf balls. A rubber center is coated with a plastic shell. The rubber is centered in the plastic injected shell. If the insert does not have to be totally engolfed within the plastic a simple insert molding process can be used.
Hope this helps