You are moving in the right direction. I would contact Attorney George Morancy for some guidance. He is one of the few attorneys I've worked with that know the BRA and ISD inside and out. You might also have plans already filed with the original unit deed, making an architect unnecessary but you never know what answer you'll get until you file. You can contact me at firstname.lastname@example.org for George's contact info.
If you cannot do it "as of right" you may still be able to obtain a variance. There is a process to it and an attorney can help you navigate both the BRA and ISD. It is not a clear-cut process and many property owners get tripped up in the technicalities so an attorney that specializes in zoning is paramount.
I hope this helps!