First, welcome aboard!
This 2' X 4' X 1/2" plate you're trying to add... What alloy is it? Do you know? It sounds like this piece is cast steel, or even cast iron, with the "bubbling" you describe. 1/2" plates of cast steel or iron, planchard-ground to a finish that would make you think it's regular cold- or hot-rolled steel, are common for extension beds on machine tools, like a vertical mill, or even the table of a saw. I'm wagering you're dealing with something cast, not wrought, and it will be a fight to combine the two.
It can be done, but in this case (if I'm right), I'm not sure it should be done.
Other opinions will follow shortly, I'm sure, after which I may give some input on combining the two.