You can do this online without problem, but VMware recommends do this operation on non-business hours if LUNs experiences high I/O.
VMware KB: Growing or expanding a VMFS volume or datastore
Note: If the LUN experiences a high I/O throughput when growing the VMFS, the ESX host may not be able to complete the operation. In such a case, repeat the process during non-business hours and when backup operations are not running. If the problem persists, power off some of the virtual machines residing on the LUN and then retry.