I have tried below command to append some path to system path variable by batch-file :
setx PATH \"%PATH%;C:\\Program Files\\MySQL\\MySQL Server 5.5\\bin\" <
To piggy-back on @Endoro's answer (I lack the rep to comment):
If you want to change the system-wide environment variables, you have to use /M, a la:
setx PATH "%PATH%;C:\Program Files\MySQL\MySQL Server 5.5\bin" /M
setx.exe is picky about placement of the /M, BTW. It needs to be at the end.