Yes, you can add a mattress topper to a mattress like the AS5 Hybrid. A topper can alter the feel of the mattress, making it softer or firmer depending on your preference.
However, it's worth noting that the AS5 is already designed to be quite plush, so adding a soft topper may not be necessary. I should also point out that the topper will make this 14-inch mattress even taller, which might be difficult for some sleepers with mobility issues.
If you do choose to use a topper, ensure it doesn't interfere with the mattress's cooling properties or compromise its overall support.