I’m not sure I’ve understood quite what you’re after @vdongen
If you try to copy a transition between two clips and paste it between two other clips, the transition will still display the video/image from the first two clips, thereby it’s not going to work.
Every transition in the Shotcut function is a unique blend of the two clips, so in general terms it’s not possible (AFAIK) to merge clips in a batch, however there are dozens of presets in the transitions list, so just picking the same preset with the same length of transition for each instance will make them uniform.
The easiest way I have found to make sure transitions are uniform is to place the playhead at the appropriate place in the preceding clip (say 20 frames from the end assuming 30fps or longer for more gradual effects) using the cursor keys for fine control (a quick press of the cursor key moves the playhead one frame in either direction) then drag the following clip in to meet the playhead where it should stick like a magnet. Do that each time you transition between clips or images and choose the same preset and Bingo! Nice, even, uniform transitions. You can also drag the edges of transitions to alter their length once put in place.
There are other ways of doing transitions using specific transition clips utilising chromakey, clips with their own alpha transparency or by using fade/opacity filters but for the built in function the method above works best for me and I often use dozens of transitions in even short videos.
The short clip below is an example of a simple transition done with opacity filters and a plain white screen which is discussed in the following thread:
