There are a couple of options.
1. Homeseer zwave retrofit motion sensor
Homeseer has a Z wave sensor that you can add to your existing fixture. It can also turn that light on or off. So it’s designed for exactly the use case you describe it. It’s just kind of ugly.
That’s the least expensive method if you want each light to have its own sensor.
Other Options
The alternative is smart bulbs in a regular light fixture, or a regular smart switch with dumb bulbs, no sensor, and then mount a separate smart motion sensor, however many you need to cover the zones you want to have.
2. Hue Bridge, Hue outdoor lights, Hue outdoor motion sensor
If you use Hue bulbs with the hue bridge, you can use the Hue outdoor motion sensor to trigger the bulbs or trigger them from SmartThings. But getting smartthings to see the motion sensor gets tricky.
3. Zooz outdoor motion sensor
Another option with easier integration is just to use the Zooz zwave outdoor motion sensor. It works quite well. Again, you could either use smart bulbs in dumb fixtures or use dumb bulbs with smart switches.
4. Ring Floodlight Cam
There are a lot of ring models with similar names, but one that does have SmartThings integration is the floodlight combined with the camera combined with the motion sensor. This does have an official SmartThings integration. If you also want the camera, this is a very nice option, although too expensive for just a light.
https://www.smartthings.com/products/-/filter/brands/ring
5. Ring Security Lights
The other alternative is the ring lighting system, but it only has partial integration with SmartThings. See the battery light discussion, although they come in a hardwired version also. Since these models don’t have a camera, they are competitively priced with most of the other options.
So there are quite a few options, but if you’re OK aesthetically with the homeseer, it’s going to be the most straightforward ST-compatible replacement for what you currently have. 