i do plenty of loans in Westchester... but i am not certain on what county and city restrictions there are, if any to do what you are asking.
if the property is next door, you will need to get city permits and check with the county recorders office regarding the construction of combining two properties into one. I HAVE seen this happen before many years ago. Check for City Permits and County recorders for building, constructing and combining your new homes into one. Those are your first two sources to check in with.
Next and secondly, , you need to make sure that the lender on your EXISTING loan is capable to lend on the new property with their guidelines, if they will qualify you for the adjacent home, and if they will write and fund a mortgage on that new, adjacent property.
Third, you may want to take a look into some options with your current lender and then several other firms who are qualified as a Direct Lender / Mortgage Banker (specifically), who all have Broker sources for niche products, like RPM Mortgage is. Your loan seems to likely fit into one of the niche products we have.
Basically, Ask a few other direct lenders for 2nd opinions on what your current mortgage holder says.
the lenders you choose to speak with which are hopefully experienced enough to give you the details on financing the additional property and structuring your mortgage loan that would be eligible for.