If you declare them, I would think, yes. If you don't declare them, they could find them if they happen to search your bags. If they're not in store boxes with all tags/stuffing removed, laced up etc you could say they are used. Even wear them around the house/yard to get some dust/slight scuff on the treads. I think customs concern is that you'd be bringing them in (new) to sell them. You could say they belonged to a relative/friend who doesn't wear them so you brought them along to give away....a lot of people buy shoes and never wear them or only once or twice if they're not comfortable.

I'm no expert, but those are my thoughts.