duct: tap into a transition fitting

[HVAC-Novice]

I have a large transition (72"x100 - 54"x38" and 30"loong). I don't really have straight duct up or down-stream of that. 

but I need to tap another small 12" duct into that duct. In the real World they can easily just tap the small duct into the large transition. but how do I make that transition behave like a duct when it comes to tapping? 

My sloppy solution is to just run the small duct into the transition and nothing connects. But that will not provide me correct airflows. in this case, the small airflow is not very significant. but in some scenarios it could be. 

One thread suggested to manipulate the transition and add a connector. But that seems inflexible considering I just slap a duct in at seemingly random locations. 

[RobDraw]

I've addressed this with one off equipment families. Hasn't come up enough to advance it to a parametric family.