I just tested it with a stock transition that I modified. Just put the cursor on the timeline where the transition occurs and COPY, then you can PASTE EVENT ATTRIBUTES to all the other transitions by right clicking the timeline, choose SELECT EVENTS TO END, then PASTE EVENT ATTRIBUTES will add your transition to any transistion left in the timeline.
Yes, you *can* save your new transition.
When you create or edit your special transition (adjust values) in the Video Event FX window .. at the top .. is PRESET: (with a menu box and, to the right side, a down arrow to show your choices)
Enter the new name for your special transition and click the floppy disk icon and your special transition will be saved. It will show up in this menu, but it will also show up in the Preset: graphic window with full animation like any other transition.
No, it's not exactly like a macro and it's not really covered in the help files. There are many scripts out there you can use as examples. But it would take a very specific script that would add the effect and then keyframe it at the proper points and then add the transition for you. It would be a highly specialized script.