[NTG-context] character spacing for guillemets in English

Is there a global way to insert a very thin (ca 1pt) unbreakable space before a right guillemet and after a left guillemet when the main language and character spacing are English, so that “«” and “»” in my source file are processed properly? The font that I am using puts them too close to the enclosed characters (typically, Greek). Basically, I need to borrow the French spacing for these two characters but have not been able to figure out how to do this from the ConTeXt source files. Alan