I think you are asking about adding directories to Datanodes.
dfs.datanode.data.dir in the hdfs-site.xml file is a comma-delimited list of directories for where the DataNode will store blocks for HDFS. Plus, https://community.hortonworks.com/questions/89786/file-uri-required-for-dfsdatanodedatadir.html
Property
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Default
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Description
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dfs.datanode.data.dir
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file://${hadoop.tmp.dir}/dfs/data
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Determines where on the local filesystem an DFS data node should store its blocks. If this is a comma-delimited list of directories, then data will be stored in all named directories, typically on different devices. Directories that do not exist are ignored
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Otherwise, I'm afraid your question doesn't make sense other than running mkdir HDFS command to "add a new directory in HDFS"