Generally you are going to have to replace the whole crank set for a double since the existing star has no provision to mount an inner ring. Apart from the mounting holes, the machined surfaces /spacers have to be positioned precisely to effect the correct spacing between the inner and outer chain rings.

Also, if you do change your crank set, be advised that you could mess up your chain line in the process - and if you want to stay cottered ( and I think you would on such a bike) finding a suitable spindle could prove to be very difficult.

My advice would be stay with the single crank you have. - If your bike is anything like the PNC-40 you referenced (picture (below) you really want to leave well enough alone.

Besides, if your bike has a chain-guard you will only be creating interferences.