How do I map www.whatsinmyshop.com to www.sheepwalkhosting.com/shop

I own http://www.whatsinmyshop.com and I have a WP multi site at http://www.sheepwalkhosting.com. I would like people to go to http://www.whatsinmyshop.com but really go to http://www.sheepwalkhosting.com/shop and still see http://www.whatsinmyshop.com in the browser address. I would like new sites people create to be http://www.whatsinmyshop.com/mynewshop and not http://www.sheepwalkhosting.com/shop/mynewshop and for them to see http://www.whatsinmyshop.com in the sign up page.

Sorry for all the information, but I do not know how to proceed. Perhaps I need the Domain Mapping plugin?

Many thanks
Marc