how to copy full matrices into bigger ones?

I have a let’s say "jit.matrix something 4 char 300 200" and a "jit.matrix bigsomething 4 char 10000 10000" and i’d like to copy the small one into the big matrix from position 400 500. The result would be a big 10000×10000 pixel resolution white image with a 300×200 small image in it.
jit.glue doesn’t let me set the position and the algorithmic way of getcell – setcell is ulra slow so i’d need a way faster method for my project.
Thanks in advance. I hope this is something obvious i overloked in the tutorials.