is there any way to make the player crawl (swim on land) without replacing blocks?

as the title says, i wanna know if i can make the player crawl without replacing either the top or bottom block they take up. there doesn't seem to be any procedure blocks for it so i'm open to messing with the java code if that's what i have to do (if it matters, i wanna cause the player to crawl when they walk on a block)

thanks in advance ^^