After a meditation retreat I noticed a 'breathing' sensation on my hands, soles of my feet and at my crown chakra. Can anyone provide more information regarding breathing via skin pores? I'm looking for exercises to expand this breathing so all of my body is covered.

Thanissaro Bhikkhu provides lots of info and tips on working with these sensations. If you go to dhammatalks.org you will find buckets of short talks (10-15 min) most of which will touch on this topic in one way or another (example). Longer talks at http://www.audiodharma.org/teacher/16/