You can do this using web browser on the phone but I realize in app would be ideal. I plan to make a super fast in app workout editor but it might just be a simple creator that will create new workouts but might partially solve this.
We actually have 2 ways to modify your workout as you are training.
Reducing length or increasing length in general are super easy with skip and extend as well as the auto extend setting. You can change your default interval extend interval time by holding it down. +10s can be +5m for example. Then using + - you can set the desired intensity.
Assuming you are talking about increasing the number of intervals in a workout there are two options for this. I actually find getting good at this process makes it easy to dynamically change the workout but if you want it all automatically setup before you start training I realize this is not optimal.
#1 Use Extend and slope mode
Just press +5m as many times as you need when you get to your last interval or where you want. Then switch to slope mode and do old school intervals by using gears. Then when you finish your intervals you can switch back to ERG and press skip to get to the end of the current interval and continue your workout as it was.
#2 Dynamic ERG creation and adding intervals
Build some “interval blocks” in your library and what you do is when riding your main workout you get to your last interval and you go to the library select a block and merge it into the current workout and get more intervals from that block. Your block might have 5 extra intervals and between skip, extend and ± you can fairly quickly reproduce the original intervals.
But still your request is valid. Just giving some ideas if you were not aware of them.