adding rear V-brake, when there are no bosses...

Good welder

To be brief... No.

You can have custom mounts machined. Have the mount be a two piece that is held together by bolts, and it clamps onto the frame. But it seems like it would be a major headache.

depending on the material of the rear stay, you could possibly add bosses by welding some on as thats how they would be made in the first place.

but the issue i would consider most is whether the frame would be able to take the forces of a v-brake when it wasn't necesaarily designed for. that part of the stay would have to be strong enough to resist the rotational force of the wheel when braking, and also the pushing outwards force of the brake levers pushing against the side of the rim. You can visually see my steel hardtail frame flex outwards when you pull the rear brake hard.

There are after market clamp on brake bosses availible. Can't think of the manufacturer at the moment.

This question has been asked many times before, you can either buy it directly from Pace Cycles or if you are in no rush you can pick up a used set on on Ebay for around 10 pound.

https://www.pacecycles.com/product.asp?catID=3&subcat=439

I hope you find this useful.

An alternative if you have a steel frame would be to use a Magura rim-brake with the braze-on mounts. I guess you could get a similar mount machined in aluminium and welded on if your frame is aluminium and you know a good aluminium machinist/welder.

Is it new?

If the bike is AL, adding bosses is a no go.

What type/age of bike?

If its Alu, really, dont even think about messing w/ it. Your better off jamming your shoe in between your tire and frame to slow down.