they are bolted together... there is probably some sort of sealant to make sure there is an airtight seal.... You should be able to change backspacing by switching out the different halves but it might be cheaper just to buy some with the right BS, i am sure it will be expensive to buy the rim pieces and then have them assembled.
And alot cheaper than one would think! (I just called this week and asked this very question myself).
All they do is change out One hoop if you don't plan on changing much, and they will even ask you if you want the change to be on the outside or the inside! Pretty cool when you want to fine tune your set up.!!
I would recomend sending it back to the original company who made them and simply tell them what size you want them to be.. That is the beauty of owning 3-piece wheels! :biggrin: