Yes, some hip protectors are specifically designed to be worn over your ski pants. These over-pants styles are most common in racing, where athletes need to quickly slip protection on and off between runs. They're also handy for rental situations or if you want to easily add protection without changing your base layers.
The trade-off is that over-pants protectors tend to shift more during a fall since they're not held as snugly against your body. They're also less discreet—you'll clearly see the pads on the outside of your pants. For most recreational skiers, an under-pants design (worn as a base or mid layer) keeps the pads more secure and better positioned to actually protect your hips.
If you're considering the over-pants style, make sure it's truly designed for that purpose rather than just being a regular protector worn incorrectly. And if you go this route, look for one with adjustable straps or a belt system to minimize shifting on the mountain.