Okay, I think this answers your question, but it's a little hard to explain what I'm trying to say here. I think that compared to your sleeve, the standard shinigami kimono sleeve is shorter in two directions. It usually falls to about mid-forearm, and the circumference of it is quite a bit less. Forgive the atrocious MSPaint (my computer with Photoshop is acting up a bit), but this is (again, in my opinion) what it should look like.
Now, I'm assuming you want to minimize the sewing involved, so I think you should try to preserve the front edge of the sleeve. Basically, you should first detach the sleeve from the shoulder seam. Get rid of that skinny sleeve portion of the main part of the kimono, and a good chunk of the sleeve itself
(measure it against your arm before you remove the sleeve, to get the right length). Then trim and sew the sleeve so that it's not as big around
. Tear out a few inches of the main kimono section and tuck the edges over and sew
. Now attach the sleeve to the main kimono
along most of its circumference, but leave a slit
just like on the kimono.
Again, sorry if it's unclear. I hope it helps at least a little!