The extender mentioned is for the BDL-style (hinged floorplate) 700s. I don't know of any extended magazines for the DM-style 700s, but maybe someone else has seen one.
(I suppose it might be possible to convert a DM to a BDL...not sure how much more complicated it would be than changing out the bottom metal and doing any required inletting on the stock, though I suspect the BDL has a smaller footprint than the DM bottom metal...this is really just thinking out loud here, since I can't hop up and go compare the two...)