Berry,
nothing is ever that simple! You would have to be senior enough to switch over to pax FO. Ie, every SO that joined before you would need to be upgraded first. So assuming that happened between 3-4years, then you can apply for FO position. If they are short of freighter FOs on your base, they will keep you there but pay you FO pax salary...not sure if they would pay you HKG FO or US FO but suspect the former. Eventually they will let you onto pax fleet. Not sure of the basings situation for FOs in US so someone else will post their opinion.
HKG FO4 earns $810K HKD pa(plus maybe 13th month but not guaranteed). Exchange rate pegged at 7.8 to the USD. US pax FO4 would earn $113K USD.
I would assume you would have to move to HKG but you never know with basings.
good luck