"Cold Formed" means that they have been bent without heat from steel sheet stock. This would form strain-hardened areas about the corners of the bends which have significantly higher Fy than the base steel. That, however, does not automatically mean to say that the design capacity of the joists depends on these cold worked areas.
Cold forming can be done with plates to significant thicknesses; If I remember correctly off the top of my head AS/NZS 4600 says up to 25mm thick.
If you have a plate thickness of 3mm or more, I would say welding is possible. That doesn't mean to say you can definately weld. You probably need to consider the application and see if the base steel strength without cold working could span with the applied loads in your application.
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...