If you are able to compile c/c++ code on your Raspberry then just cloning the nim repo on the Pi and following the build steps should be enough.
You can also cross compile the code. I do this for my beaglebone (arm) library here: http://forum.nim-lang.org/t/1318#8315
This is easy for the BeagleBone as this board uses the default kernel. As far as I know the PI community change the kernel so I don't know how easy will be to cross compile.
You might be hitting this issue (Closed): https://github.com/nim-lang/Nim/issues/1934
Maybe this will help:
Araq: "So use cc=clang in the nim config file?"