If you already using both MinGW and MSYS installed by you, and these are working well, You should copy to each directories manually.
copy MSYS files
MSYS files and directories are below:
bin # directory for MSYS
etc # directory for MSYS
include # directory for MSYS
lib # directory for MSYS
m.ico # Icon for MSYS
msys.bat # MSYS launcher
msys.ico # Icon for MSYS
postinstall # directory for MSYS
sbin # directory for MSYS
share # directory for MSYS
You just copy and overwrite these files and directories into your MSYS directory except fstab, profile and msys.bat. You should merge these 3 files carefully. For example, You can write MinGW's Path in fstab file.
copy MinGW files
MinGW directory and DevKit installer file are below:
dk.rb # devkit installer to lib/ruby/site_ruby
mingw # directory for MinGW
You just copy and overwrite these files and directories into your MinGW directory. Then you launch DevKit installer. If your MinGW and MSYS both work collaboratively, You can success install DevKit.