I'm an architect and do real estate rehabs on the side.

@Jared Stroebele I would consult with a local builder or architect but my understanding is that it could cause cracking of the slab.

Architect Search - Afternoon BP community.  

I know we have to visit the architect to get a feel of the cost; but has anyone done conversion form STR to MTR before and what are the pitfalls? 

Jim - How did you get the "Architect from..." after your name?

When I'm wearing my Architect's hat, I find this to be one of the top 5 "oooops...I didn't think about that" items when doing rehab.  

Generally, an architect should suffice and your city probably requires one for changing a layout.  

If you were designing a building in NYC, which brand name architects would you hire to command the highest price point?

You'll then need to have your architect accommodate those requirements.

One possible option is to drive light wells in various places (you loose space but gain units) to provide the core units with both some light and alternative egress / exit (fire code in many places).Make sure you have a good architect / designer who knows local code before you go to far.