The tech tree is a part of GameUI (since patch 3.0 I believe). So, editing the panel via UI xml code should be possible. This would be required, if you want to add new structures into it.
However, I do not know, if we can add TechTreeStructureFrames. They might be flagged as Blizzard-only. So, the worst case scenario would be putting the data into the images and labels yourself, which should be possible.
The unit names listed at each structure are taken from the unit data in data module. Edit the "Tech Tree Unlocked Units" and the "Tech Tree Produced Units" fields, if you want to add/remove units listed.