Brass Floor Lamp

Is it possible to weld or epoxy so that I can paint it?

depends on what type it is a case then you might Soder meatus or welded, then sand and paint the differance you do not, but whether it is a cerramic then you can go to the store and buy the house Ceramic Repair and Improvment box telling you how to do that

if it is brass may be soldered.then painting

You can try to solder, but do not use more heat than necessary, just in case it is brass plated and not solid brass.
Also you can epoxy the back to hide the repair. Then use a steel wool pad, rub in the same direction as the crack to mix in.

since you said it's an antique, I suggest you consult the antique restorers. If you try to do it yourself you risk destroying the value of it.