It is legal for the construction firm to employ, in-house, an architect. It is legal for the architect to stamp his/her designs while employed by the firm. The work of the architect, no different than in-house legal counsel, rides on the license that the architect has with the state. There are no other licensing requirements that the construction firm must obtain...though it is not a bad idea as a matter of due diligence, for the architect to report who he or she is working for to their licensing board.
The architect can only stamp their work...not the work of other unlicensed professionals.
One other point of emphasis, it will be a necessity that the construction firm pick up a malpractice type, errors and omissions, insurance policy to cover the work of the architect for the firm.
Here is a flier produced by the State of New York that covers some of these issues:
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Best wishes going forward!