Well, the problem lies not within SIM. We have to develop an application which will switch from one operator's network to another network within the same operator.
One thing is almost sure. It can't be done with SDK's APIs. You have to get deeper, probably to the radio module, where you can send AT commands directly to de/register to a network. But that would mean you have to build a whole platform from scratch and there's no help on sending AT commands out there in Google's knowledge base.
If anyone has tried that, I'd be grateful for a piece of information on the topic.