@Aerilius took your question to mean “can I run a SketchUp Ruby extension without running SketchUp”. Since an extension makes calls to the internals of SketchUp, the answer is obviously no.
But if you mean “can I place SketchUp Ruby scripts in a non-standard folder”, the answer is yes - though some “bootstrap” script must be run either from the SketchUp Ruby Console or the standard folder to load them. SketchUp only knows by default to search its standard folders for Ruby to load.
Various extensions have used ad-hoc, non-standard folders in the past, though usually to store configuration or other customization data rather than Ruby scripts themselves. However, due to bad experience and maintenance issues this practice is now generally discouraged.
This raises the questions: what is your use case? why do you want to do this? Maybe there is a better answer…