While sitting cross-legged might feel cozy for a while, it’s not the best position for your posture or circulation long-term. Ergonomic chairs are built to support a neutral sitting position where both feet are flat on the floor.
That said, it’s totally okay to shift into a cross-legged position from time to time, especially when your body needs a break from the usual. Just be sure to return to a supported sitting position with both feet down for most of your workday.
An ergonomic chair gives you the flexibility to move and reset, that’s part of what makes it so effective. So yes, you can sit cross-legged in the right ergonomic chair, if it’s done in moderation and the chair supports it comfortably.