Actually you can't just add it. For rented property you need
Dwelling coverage.
Your homeowners insurance policy becomes automatically null and
void when the property is rented out to another. You can have your
agent endorse your policy for rental dwelling coverage or you can
have your agent re-write the policy on the appropriate dwelling
coverage form.
Homeowners insurance is for owner occupied homes. Dwelling
insurance is the landlords insurance for rented properties