Hi Mohammad,
SAP doesn't have a ready made CAD system, however it has needed integration and User Interface software to integrate CAD systems. SAP Engineering Control Center is one of the recent and best interfaces ever produced. It comes with predefined interfaces for 8 best CAD systems (could be more), you can find more information here ECTR Wiki , sap help ECTR page and a short overview here on my blog.
It also has CAD Desktop (cDesk) to integrate CAD systems. You can find more information on it here and look for CAD desktop or CDesk. Here is the sap help link for the same.
I would suggest to look at ECTR option as its really easy to install and provide an intuitive User interface for the CAD users to integrate with SAP.
Hope this helps.
Thanks,
Amar