I am owner of a vintage peugeot bicycle. I'm not yet sure about the model. I tried to find some help on identifying it in this post: https://www.bikeforums.net/showthread...identification.
It has cottered crankset with only one chainring and without any holes for bolts for a second one. It has five arms and is similar to the one in this photo, without the holes: https://images.andale.com/f2/115/106/...tooth_ring.jpg.
Is it possible to add an additional chainring someway or I'd have to change the whole crank set?
Any advice could be helpful!

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.

The bike doesn't have a chain-guard but it has a braze-on mount point on the bottom of the vertical-seat tube. I think that is for a front dérailleur that is missing too. That's why I want to add an additional chain ring. How could I find out the purpose of that braze-on?

can you show us a photo?

Ditto. I'm very curious to see what you have. Plus, it will help us to help you out.

I have no photo of the bicycle.. It is identical to the PNC-40 shown above.
The mounts for the pump are on the down-tube.
It is purple, with different saddle,pedals and tires, the lights and the chain-guard are missing, and the crank set has a slightly different motivo with 5 arms.
For more technical specifications look over here .

How about you take a photo of the bike, if you own it. The lustre of old paint, the glint of chrome, the smell of leather...well maybe not.

We want to see it, not read tech specs.

It doesn't sound like a bike that should be modified.