Plastic beads can often be set into polymer clay and baked in place, particularly if they're placed kind of *down into* the clay so they're protected a bit from the heat (you usually won't need liquid polymer clay or any other type of glue if the gems or other inlays are pressed deeply enough into the clay) . Different types of plastic have different distortion or melting points though, depending on the exact type and formulation of plastic, and some may also change color or become cloudy with sufficient heat, so you'll have to test yours.

Of course, it's best to make sure the temp reaching the clay is actually not any hotter than you think it is too, so it's important to use an oven thermometer, know where any hot spots might be, use a baking surface that doesn't get hotter than the surrounding air temp, and at perhaps to use one of the partial-covering or complete-covering methods for baking polymer clay.
You can read more about all that baking stuff on the Baking page of my polymer clay "encyclopedia":
http://glassattic.com/polymer/baking.htm

If you test your particular plastic gems and find that they do distort/etc, just do them as mentioned above by making impressions where you want them to go, then glue in after baking (superglue, white glue esp. one like GemTac or Jewel It, E6000, 2 part epoxy glue, etc.). Here's another tidbit from my site about doing it that way:
"When I've used the cheap plastic rhinestones, I glued one onto the end of pen cap so I could make an impression where they would go in the finished product."

You can also "onlay" your gems by creating polymer clay bezels/frames around them without even pressing in... check out some of the examples on this page:
http://glassattic.com/polymer/onlay.htm ...look under the Dimensional Onlay category, especially under Ropey, Bezels, etc.
...crud, most of the owners have moved their pics breaking my links (though I still have more info on the page), but here is at least one:
http://creaplastic.free.fr/03_04.htm
And you might also get some ideas from the *Very Small Frames* on this page:
http://glassattic.com/polymer/frames-mirrors.htm

P.S. Are you aware that Sculpey III is one of the 3 lines** of polymer clay that will be weak after baking in any areas that are thin or projecting? (rounded, fat shapes are inherently strong).
FimoSoft is stronger, and all the others are quite strong when thin and will only bend if thin and stressed (Premo, FimoClassic, Kato Polyclay, Cernit, SuperSculpey-Firm, etc.).
Those 3 Sculpeys are also quite soft when raw so aren't preferred when one wants to do crisp detail and want the clay not to distort (and get fingerprints) while working on it.

**Sculpey III, SuperSculpey-flesh, and original Sculpey (which is the worst for strength)

HTH,

Diane B.