Really the only way to do this is if you go into the UT Aircraft Assignments and assign the B717 to the 146 equipment. If they still fly the 146, you can leave the 146 there and UT will use both the 146 and 717. You can even limit the number of times UT uses the 146, which means that it would use the 717 more.
If you have your own flightplans and you know how to add in flightplans, you can always go into the Compile Options and deselect Qantas / Airlink and use your flightplans instead. Remember if you make any changes, always compile! I'd suggest visiting the Ultimate Traffic forums at http://ultimatetraffic.flight1.net/forums/
which is where I usually hang out as well as a bunch of others that can help with this and other UT questions.