[NTG-context] stretching/expanding Greek characters

I am trying to typeset some compendia found in a medieval Greek manuscript and would like to stretch or expand several characters in my font. So far, I have mostly found information about stretching the inter-word spacing (tracking). There is, however, discussion in a few places of what I want in Arabic but I am at a loss to see how I might adapt this to Greek. If this is even possible, I would be grateful for any suggestions or pointers. My Greek font, defined as a fallback, is IFAO Grec Unicode (regular only). Alan