Harryshin, I'm not sure I understand your question, but if you want to go to Harvard directly out of a bachelor's program, I don't think USNA is for you.
I think you're asking about VGEP (Voluntary Graduate Education Program), which sends about 20 graduating Midshipmen directly to master's programs @ cooperating institutions. When my son (class of '11) entered VGEP, it was limited to DC area schools -- Johns Hopkins, Georgetown, GW, U of Maryland, American, etc. He's treated as active duty while studying (ie, paid), but VGEP did not extend his military commitment -- just delayed its start. VGEP is very competitive, and only seems possible if you've waived a couple USNA classes and/or taken summer credits. Son actually started his master's classes during the 2nd semester of his 1/C (senior) year @ USNA; not certain, but I think that timing is the norm (which would explain limiting it to DC area -- he still had a couple USNA classes as well).
If you're talking about entering a grad program after you've completed your initial military commitment, yes, it would extend your service. I think the rule of thumb is 2 more years of service for every year of education.
There's also IGEP, Immediate Graduate Education Program, but that's strictly for the nuclear submarine and surface communities @ the Naval Postgraduate School. Also a very small number of graduates go directly to medical school, but I think you'd have mentioned medical training if that's what you have in mind.
You may find some answers here: http://www.usna.edu/AdminSupport/Not...ICE%201520.pdf
... or it may just leave you more confused.
If I didn't answer your question, would you restate it?