You can now create your own Google Expeditions using Google Tour Creator. Expeditions support was added in February of 2019 for Android and March 2019 for iOS.
In this video I show you how to send Google Tour Creator Tours to Google Expeditions. In this video I also use a Teamplate (which are also great ways to see how immersive Tour Creator can be) but you can create a tour easily from scratch! If you are new to how this is done I recommend you check out my video on how to make a tour in Google Tour Creator as well as my video on how to make your own 360's in Google Street View.