You can add them from your IDE, but then you have to go back into Projucer and call “Add existing files” and re-save the project.
Or like I usually do it, call touch Source/MyFile.h and select the file in Projucer using “Add existing file” and save the project.
Having our own templates would be lovely, you can get very fancy there including reading the base class and offer virtual methods pre-populated… but for time being there are alternatives.
Yes, with a decent amount of configurability. I do custom development for clients and being able to have e.g. a correctly formatted copyright header for the client whose project I happen to be working on at that moment would reduce daily frustrations.