[John Perez] “I want to load my fave transitions on that dropdown so I don’t have to go through the step of insert other”
I don’t get a drop down menu like that. What version of Vegas are you using?
The bottom line is that things like this can’t be changed by the user.
Here’s what it looks like when I try that in Pro 12.

“have to manually choose the same transition over and over again”
To do this to multiple events simultaneously, apply a transition to the first event, click it, hold down the Ctrl button and click on all other desired events and then drop the desired transition on the first event. All selected transitions will now be the same.
Make sense?