Hi, Yanky,
I'm not sure I understand. If the mag is pre-ban, it does not matter what gun it fits, or how many rounds it holds, or how it is altered.
If, let us say, I want to alter a WWII 30 round TSMG mag to work in an M3 SMG, I don't need approval or anything else from BATF. (Yes, I know it probably can't be done - this is hypothetical.)
If I take 3 10-round post-ban mags and weld them together to make a 30-round mag, I am violating the law.
Jim