The Freedom 458 comes in several models -- 10, 15, 20, 25, and 30, and then some of these have an S/D or D/D option. There is also a "combi" model. I need to know which of these you have to help.
Most likely, you will hook your system up per the diagram on page 43 of the manual. However, the presence or absence of a bonding relay depends on which model you have.
If you hook the unit up correctly, there are no issues with applying external power while the unit is running. Remember, there are separate input and output connections -- never, ever connect an external power source to the inverter output.