You can edit your listings to add a message. I don't know if you can do it in bulk, as I would never do it (I believe if you are open for business, you are open for business).

HOWEVER, as you are doubtless aware, most buyers don't read descriptions. So when they are asked (for untracked) if the item arrived on time, you can expect a few No's. And possibly non-green dots. Any parcels that don't have a scan within your handling time will get an automatic late postage defect from eBay. Too many of those and you might find your selling restricted.

You could extend your handling time, but that won't stop non-green dots.