You may not be able to get any AA agent to book it the way you want, since they are increasingly monitored about not breaking rules, and the tools they are forced to use won't let them bypass such things. You can try asking them to book flights with six hours in JFK, since on a domestic set of flights, that should be sufficient to be a stop and not a connection. Or you could try adding an extra connection (e.g., JFK-DFW-MIA) and when you get to JFK, ask them to reroute you on a nonstop JFK-MIA.