You would need an automotive fabricator...like LA Street Customs... to squeeze a 3rd row into the e53's cargo area. The e35 & e70 X5s are two different generations, platforms, length, body design, etc. No way is it a plug-n-play or unbolt/re-bolt proposition.
The e53 was never designed to even consider a 3rd row...and trying to bolt on the 3rd row from a later design would need fabrication to even attempt to get it in there. And you've given no description of the e53 since some have a slide out cargo floor and some don't...but I guess it doesn't matter because the e70's 3rd row seat isn't an easy swap anyways.
The e70s with 3rd row seat has a different cargo floor layout (no spare etc) to accommodate the 3rd row. The e53 has the spare and components for the air suspension (if 4.4/4.6is/4.8is & optional on the 3.0) as well as the box that contains the battery and other components.
Here's a visual of both cargo areas...you can see that the e70 w/3rd row seat has more square cu.ft.:
{E53 X5 Cargo Area}
{E70 X5 w/3rd row Cargo Area}
Bookmarks