You're asking about the rim. I'm assuming you'll be keeping the stock hub? The XT225 has 36 spokes in 4 groups (different styles) of 9 spokes each. I'm also thinking the XT225's rim is "dished" slightly. So you'll need to check the number of spoke holes in the 250 rim. Needs to be 36. Also that the spoke nipple holes will accommodate the XT225 spokes. They have limited adjustment too. If the profile on the rim holes is different that may prevent mounting spokes and tensioning them properly.
If you're looking to beef up the XT225's rim there are other options. Excel makes good rims. A company called Buchanan's Spoke & Rim, Inc.
can create a solid rear wheel for you. Better spokes, quality rim and a stronger lacing pattern. And off the stock XT225 hub.
Keep in mind the XT225 hub is a molded metal process and somewhat soft. Causing spoke holes to elongate. The shoulder on those spokes is also sharp as opposed to a straighter pull. Further weakening the build. Butted spokes can help there. Good luck and I hope this helps.