You mean before NS? I d not think it is advisable to do that b4
NS since work does not get you deferment from NS.
If you are talking about after NS, then chances are you might be
too old for the SAF to consider recruiting you already. eg: 21 ORD.
5 years FFL = 26 years old.
In addition, I do not think the SAF will give you advanced standing
on the basis of your prior experience. You will have to start
again.