You could, but you're always better off adding leafs that have the same arch and thickness as the existing pack. That way, the leafs will flow better and actually compliment each other.
Adding leafs that are a different thickness or different arch as the rest of the pack, creates a conflict within the pack, and the result is often poor ride quality, poor flex and reduced life.
BTW, most new leaf packs will settle slightly after a break in period. On an XJ, you will usually see this happen on the passenger side first, since the torque load is transferred to that side. Usually, you can just switch leafs from left to right and wheel it again. Often times, you'll be sitting level again- unless the sagging leaf is really shot.
Of course, there are some who install a nice set of leafs, and then add some blocks and longer shackles but don't add more bumpstops. You CANNOT expect a leaf in that situation to compress as far as it did when stock and be able to live. I know a good number of guys are running 6-8" lifts with 31" to 32" tires, no extra bumpstops, and wonder why their springs are sagging...
