To get RS232 from an F9P, see this discussion here:
It shows how someone used an RS232 converter connected directly to a ZED-F9P. Then you can feed that through an RS232 cable to the Ag Leader monitor. You have to configure the F9P to output the messages you require, and at the rate you require, and at the right baud rate. It may require a null modem adapter between the Ag Leader and the RS232 converter, depending on what gender the port is on the Ag Leader.
As for configuring the Ag Leader to steer, I do not know how to set that part up.
Without a base station the F9P probably will only get you maybe 25 cm accuracy tops. Certainly it won’t be better than the Ag Leader is for you currently. But with RTK, the story is quite different.