Here you can find a discussion (https://forums.fogproject.org/topic/6074/extending-a-file-system-on-redhat-base-distros) and wiki article on this topic (https://wiki.fogproject.org/wiki/index.php/Add_%26_Extend_a_2nd_Virtual_HDD).
Depending on your setup (you haven’t told us anything about that yet - OS, version, virtual/physical, partition layout, free space or second hard drive?) you probably need to adjust most of the tings mentioned there. But this should give you a good starting point. For further assistence we do need more information!