There are laces designed to convert a shoe meant for regular laces into a slip-on, such as https://www.locklaces.com/ https://www.locklaces.com/ I've used them

There are laces designed to convert a shoe meant for regular laces into a slip-on, such as https://www.locklaces.com/ https://www.locklaces.com/ I've used them and they worked pretty well.