Thanks for writing. I'm not sure exactly what you're asking --- if you physically want to know how, then you should be able to dig a small hole for each plant you want to add. That shouldn't upset your rhododendrons' root systems too much.
if you want to know what plants to add, that's a bit more subjective because it's a question of what will grow in those conditions and what you like. I'd suggest doing a search in our online Plant Encyclopedia at http://www.bhg.com/gardening/plant-dictionary/
---Justin, Senior Garden Editor, BHG.com