There are two types of stacking , VC stacking or OA stacking.
OA stacking:
If you are reffereing about OA stacking, you will not need any downtime. Make suer OA credientials needs to be same in all enclosures
VC stacking:
If you are doing VC staking and if the new enclosure is already in production, please be informed that all the VC informtion and cofiguration will be cleared. Since all VC's in that stacking needs to be in one singe domain.
You will need donwitme for VC staking.
But for updating firmware of VC flexfabric or enclosure you will not need downtime.