Yes, you can still attach a rear light to a helmet without a dedicated mount, but you'll need to use a strap or clip that wraps around the helmet's vents. This is the standard setup for most road, aero, and mountain bike helmets, which prioritize a clean profile and low weight over built-in light features.
The downside is that strap-mounted lights aren't as secure — they can shift during a ride or pop off if you hit a bump. There's also a real risk of cracking or damaging thin helmet vents if you clamp a light too tightly, especially on in-mold shells that are designed to be lightweight rather than sturdy attachment points.
If you mostly ride during the day or rarely commute in the dark, a helmet without a light mount is perfectly fine. Just be gentle when attaching lights, and consider mounting your rear light on your seatpost instead — it's often more stable and visible to drivers anyway.