Welcome to the Community,
using additional LUNs (i.e. extents) to increase a datastore's size is not necessary anymore. As of vSphere 5, VMware added support for LUNs/datastores with up to ~64TB, and virtual disks (starting with vSphere 5.5) with up to ~62TB. In versions up to 3.x extents were required to expand datastores. With version 4.x VMware introduced datastore growth, which allowed to increase the LUN/datastore size to up to ~2TB, and extents were only required in case one needed a datastore larger than 2TB.
There may be best practices available from different storage vendors, but in general you will simply increase the LUN size on the storage side, rescan the ESXi hosts, and finally increase the datastore, which can be done without downtime.
André