Put a standard tyre on my rim. Blew it up to check whether it would seal and found it has 2 small punctures already. Can you still add sealant and pump up as tubeless (with a track pump) or will the air going in just simply leak straight out?
If it's a big 'ole, it might not seal but the worst case scenario is you can just stick a tiny wee innertube patch on the inside- I tend to use wee 1cm squares as any smaller and they get fiddly. (it's not like an inner tube, the air pressure will hold it on rather than trying to blow it off) If it's a slit, and it's opening up,not so easy.
But most holes will just seal.
Patch the holes from the inside using standard repair patches, or a piece cut from a vulcanised rubber sheet. Make sure the area is properly clean before gluing it, and use clamp or weight to set the patches properly.
They should seal just fine. This usually happens to me when I take an old tyre that I've previously used tubeless but have since pulled all of the thorns out before putting the tyre back on.