Thanks to Thorsten Schöning. A solution has provided by him.
sed --regexp-extended 's/(.)shutdownhtml\1(\s+)=>(\s+)\1{2},/\1shutdownhtml\1\2=>\3\1someText\1,/' params > params.shutdown
See the link below for more information http://mozilla.6506.n7.nabble.com/Command-or-Script-to-add-text-to-parameter-shutdownhtml-in-param-file-td279974.html