“Each tool get’s it’s own instance of the lua interpreter […] if you define a global variable, only the tool that defined it will be able to read it.” Source.
For your own personal use, you could maybe try using the built-in “Libraries” structure. For example, look at the included com.renoise.ToolUpdater script, which does a require "renoise.http" located in %/Libraries/renoise/http/
Distributing this would be problematic though. Since XRNX packages are self contained, you’d have to write a small HOWTO informing users that they would have to manually copy files, etc.