Here are many reasons not to use Autocad to model your game objects: You have to do some work on the models to make it look good and even work properly with Unity's render engine. I see an answer below suggesting that it's totally fine to use Autocad models in Unity or model the objects in Autocad. Since you are already using Autocad, I suggest you use Maya since the UIs are now made to look similar. If your goal is to make a game, then you need to learn either Maya or Blender.
![how to import model into unity sizebox how to import model into unity sizebox](https://i.ytimg.com/vi/Yo3RbJE_xPY/maxresdefault.jpg)
You should spend that time making new models in Maya or Blender. Note that it will take time to clean the models up. Reduce the poly counts before importing into Unity.
![how to import model into unity sizebox how to import model into unity sizebox](http://1.bp.blogspot.com/__lKAPG3mt5k/TEDhnjOM0xI/AAAAAAAAAHM/-A9hNp4x0h0/s1600/Import+Normal+Map.jpg)
If you want to use your models from any of these software, export them to software such as Maya or Blender then clean the models up. Objects produced in Autocad, SolidWorks or other similar software should not be imported to Unity directly by any means. If your Autocad can export to any of these then the answer is yes.