Is there any way to edit/create new instruments to change the range of an instrument (the “red” indications) or to create a new instrument like a traditional one that it isn’t in the default instruments?
Thanks! 
You can modify the instruments.xml file that contains all the instrument definitions. I would make a backup first, and just be aware that since this file isn’t intended to be user modified, it may get overwritten with any update. In Windows you can find it under Program FIles/Steinberg/Dorico3.5. See the highlighted region below which specifies the range.
Wow, OK, thank you!!!