I believe that these are not in the code because they’re just “regular” tags; You can make any tag you want by typing any text following an @ character. What makes some of them “special” is the syntax highlighting, which makes @critical appear with a red background, for example.
So, depending on what you want to do with your priorities, you may be able to get away fairly simply here. For example:
{
"scope": "text.todo",
"completions": [
{ "trigger": "u\t@ultracritical", "contents": "@ultracritical"}
]
}
If you save this text as Packages\User\PlainTasks.sublime-completions you gain the ability to press UTab to insert @ultracritical as a priority on your task. There is some documentation on completions you can read, but in short the bit before \t is what you need to type to expand the completion, the part after it is the “hint” that you see to the right of the completion in the list, and the “contents” is what gets inserted.
If you would like them to also display differently in the document (like @critical does, for example) you would also need to fiddle around the PlainTasks.sublime-syntax at a minimum to get the tags recognized there so that custom colors could be applied.
The only thing to be careful of is your placement of the file you create;make sure you put it into your User package so that you don’t accidentally override the file provided by the package itself.