The warning you are seeing when you do an "Add Extent" is saying that the disk/LUN you are about to add will be formatted and all data erased, not the existing LUN that was already in the datastore. So for example if you had a 500GB LUN already formatted as VMFS with VMs on it, and you wanted to add a 800GB LUN as an extent to that existing 500GB datastore, then the 800GB LUN would be wiped out but any data on the 500GB LUN would remain intacted. around page 136 goes through the process...
If you took your 500GB LUN and expanded it to 1.3TB on the SAN, then you'd want to move the VM off the existing datastore and re-do the VMFS partition to include all of the storage (there is no way to "expand" a VMFS partition itself, just add extents to a datastore/volume), In that case if you have vMotion/vCenter agent license for the ESXi box and another datastore on the box with sufficient space to hold the VM, then you can either svmotion or cold migrate the VM to the other datastore, do your re-partitioning of the VMFS volume, and then move the VM back.
If you don't have vMotion/vCenter agent license for the ESXi box but do have another datastore on the box with sufficient space you can use the Datastore Browser to do the move and move back. Shtudown the VM first and remove it from inventory, then create a directory to hold it on the other datastore that you'll use for temporary space. Then go back to the datastore currently holding the VM, select all of the files in the VMs directory, then do a right-click "Move" to the other datastore. Once you have the VMFS partition and datastore re-created using all of the expanded LUN, then go to the datastore browser on the newly expanded LUN, create a directory for the VM, and then go back to the datastore browser on the temporary datastore and repeat the "move" process in the opposite direction. After the move completes, go to the newly expanded datastore, navigate to the VMs .vmx file, right click and Add to Inventory...
