Yes,sure, there is no problem with metals, only stuff which can be melted at 150 C shouldn't be inside.
In case  if you prefer to sculpt, bake and insert somethink after - you can cut a baked piece in a half ( or whatever ) , make a room for what be inside and glue with superglue or add some polymer clay inside both haves and attach them and bake again. Tip : it is easy to cut a baked polymer clay with an exacto knife when it still hot/warm

I often put in tubes and wires that I pull out after baking. Since the clay is not completly hardend before it is cooled off, it is easy while it's still warm- as zooooom pointed out.

How do you oven bake them?

I don't follow to the instructions written on the box, only if I need to keep the actual color,let say, flesh color  I follow to instruction. I usually overbake, I mean - higher temperature and longer time than recommended.Most overbaked polymer clays are more durable.To control the temperature you should use an oven thermometer ( don't trust the numbers on your oven )  http://oswaldadventures.files.wordpress.com/2008/11/292t05385_oven_thermometer.jpg  I use a kitchen oven only for a large objects,  for all others - a turbo oven  http://www.securaturbooven.com/images/gallery/777-with-extender-ring-and-turkey.jpg   One more "baking"  tool is a heat gun  http://www.canadiantire.ca/AST/browse/6/Tools/SpecialtyTools/HeatGuns.jsp when I need the job done fast  or only a local area .