As this is a "strech", there is no ATTO, only many fibers between them. Looking behind it i can see cabling is quite complicated, we are going from Controller to shelf from each side (i did it some years ago, never tried again )
We only would like to add one shelf on each side, but do not replicate . It should be seen as storage only by the "local" controler
Do i have to change all the cabling to integrate the new shelf into a kind of daisy chain, or could i use free SAS port on the controler to have this 2 new shelves plugged simply (i have 2 free ports one integrated 0B and one on an additionnal card)
For failover to work (during upgrade etc) you should still have them cabled to both controllers.
You didn't specify the ONTAP version/mode, so hard to tell you what scenario is supported for your configuration. but unless you already exceed some limit in the stack(s) - i don't see a reason why not to add them to the existing stack(s).