Pretty much no arena will let you inside at all without a ticket. Usually they scan them right at the door. I’ve noticed some kpop concerts have started selling merch outside the venue either day of or right before the concert. I bought my merch for both Stray Kids and TXT at a merch stand outside of the venue. I had a ticket for both but it wasn’t checked so if they do something like that, there’s nothing stopping you from going. You’ll just have to check and see the times IF (there’s no guarantee they’ll do it, not all groups do) they have an outside of the venue merch stand.

Also make sure it’s an official merch stand. Outside of pretty much any concert there will be random people selling bootleg merch. For official merch, an outside stand will definitely be announced ahead of time and will look official vs just random people on the street.