You do not need to shutdown and/or restart the entire cluster in this case.
First, add the new node's DNS name to the conf/slaves file on the master node.
Then log in to the new slave node and execute -
$ cd path/to/hadoop
$ bin/hadoop-daemon.sh start datanode
$ bin/hadoop-daemon.sh start tasktracker
then issuehadoop dfsadmin -refreshNodes and hadoop mradmin -refreshNodes so that the NameNode and JobTracker know of the additional node that has been added.