Beachcomber wrote:
....
The key person to talk to is your postmaster in the local post office
serving your address.
....

Whether the USPS will issue an additional address or not has no bearing
on the legal use of the domicile outside local zoning regulations. OP
could possibly get a dozen more USPS addresses but whether he can sublet
or effectively turn a house into a duplex in a single-residence zoned
area and get away with it will depend on his local jurisdiction (and to
a large part his neighbors if outside the zoning regulations).

It is simply not wise to buy a property on a premise w/o being certain
of all legal covenants/restrictions on that property. The existence or
not of a USPS delivery address isn't one of those even though it may
currently exist and an additional one might be granted.