It would be illegal, assuming you're talking about adding an AK mag to the bottom.  You'll have to wait until the AW ban expires next September.

-Troy

ATF has ruled that you can't convert a magazine in such a way that it won't fit in the gun it was originally designed for.  While the AR portion of the mag would still fit an AR, the AK portion would no longer fit an AK.  The result is that the new mag is a post-ban, restricted mag, even though the source parts were both legal pre-ban mags.

You CAN legally convert a mag to fit another gun as long as it continues to fit into the original gun.  For example: 9mm Uzi mags converted for 9mm ARs, or M14 mags converted to fit Armalite AR10s.

-Troy