Download a STEP file of the tool you want to define in Mastercam and place it in a known location. 

In your Mastercam file open the Tool Manager found in the Toolpaths tab. 

In the right click context menu select Create tool  

Select Custom tool and click Next 

 Click the Import custom geometry from a file icon then click Next 

Select the download file and click Open 

 Define the tool properties and click Finish 

Select the Green Check to exit the Tool Manager 

 Select your custom tool in the toolpath parameters – the tool will load as it was defined 

Backplot the toolpath – The custom tool will be displayed as shown