AS BS suggests, it is certainly possible. It is probably like most other things in helicopters: it depends how much you want it. Some of the bases are MUCH less popular than others for instance.
It is probably going to be a bit more difficult for low time non-SAR specialists when you are unlikely to be able to acquire the relevant tyoe rating in non-SAR work first.
I know of cases where Bristows put new co-pilots onto the S61 on the offshore oil runs for 6 months and then transferred them to SAR. More difficult when the operator only flies the aircraft for SAR.
They have to get their co-pilots from somewhere though!