Glad to have you back!
Ahem, the Workout Creator isn’t our favorite piece of software we currently have… it was built with Adobe Air a long time ago and we have yet to update it since we’ve been focused on other new features. So I’m sorry it’s not very intuitive. 
I’ve previously used the 20 Min and 8 Min tests to create mode changes for testing purposes, but it’s really easy to mess up if you adjust the test intervals at all or try to add new ones. Since you’re using the WC this means you have a desktop/laptop computer, so here’s what I’d recommend doing instead, since it’s a little easier:
Create your workout in the Workout Creator as normal, but then use the “M” key on the PC app to switch between modes on the fly. Or just start the workout in resistance or standard mode and do your all-out sprints when you want and work to hold the power targets during the other times. I hope this helps!