How to create/upload custom Snippet

Snippets are currently not extensible. However, you can develop your own Script Function Libraries (e.g., LocationFunctions) to achieve similar (if not, preferable) functionality. I say Script Function Libraries are preferable because they will reduce the amount of code in your Services unlike snippets. Of course, you'll need to make your functions as generic as possible since you won't be able to edit any of the code.

If you're interested, the "ThingWorx Extensibility" section of the Help Center has documentation on developing extensions and widgets.