If you just want to add a * mid-dialstring at random, that's not possible.... however if you have a certain dialing format that you are after with a set pattern that dial plans in either VoXalot or PBXes should help....

As I said before, when connected to PBXes, actually dialing * at any point resets the line otherwise.....

If you are a bit more specific about what you're trying to do, I'll be happy to give some ideas...