You're question is spot-on, and the answer is a definite maybe.

It is possible, yes.  But in my experience, many academic/research faculty don't consider the professional MArch as sufficient training to launch directly into a research-oriented PhD program. 

So it depends upon many variables, chiefly the sub-field of study, strength of the applicant/application, mindset of the admissions committee, and interest by relevant faculty.

Also: you'd better be able to write well.

Also: you'd better be able to write well.

Or at least be able to write densely enough to make gibberish sound profound.  Pardon my cynicism, but...............

It’s certainly possible. I know someone doing that now. No MS required. The schools will gladly take your money. Go for it. Not sure about arch but most phd programs in other fields have a built in M.Phil that is acquired along the way with the PhD. Further you should learn how to research if you plan on PhDing. So at this point you’re not looking like much of a candidate... All the information is on the school’s website and if it isn’t clearly stated there, there’s a contact/email address toward which you should direct such questions rather than asking random internet people.

^ The above reminds me of a couple of critical points:

1. Most students get all/most of their costs covered by the program.  If this isn't offered, don't go.
2. Ask yourself: why a PhD?  If the answer is anything except "the job I want requires a doctorate," don't pursue it.  If you must get another degree, make it a second masters in an allied field.

If you are planning to do a phd in History and Theory of arch. then it will be more difficult to go directly after the Masters (but it is possible)- bc typically for the application for those kinds of phd´s you will require to submit writing samples- the portfolio of design work does not matter. There are phd´s that are more design related like computation and arch where the design work of the portfolio matters and of course writing samples as well. But do not rush take your time to find the right school. And avoid paying for a phd- you should get paid to do it!