I find it most easy to make the ref point before making the objects in to a block, so select them > press D > set the new ref point, and when still selected make it to a Block.
Else, explode a Block and then do as above, save with new block name.
Or in the Block palette Select the Block, then Edit Content, make something where you want to have the refpoint,
then use Ctrl + Shift + R to relocate the refpoint by snap to the object you made for it, and then delete the snap object, checkmark for finish edit content.
V18, V19, V20, V21, 2015/ 16/ 17 Pro. Platinum
Deluxe V20, V21, 2015/ 16/ 17
Windows 10 Home Premium 64 bit, 32 GB
NVIDIA Geforce GTX 780m, 1 GB