How to import PCB 3D model (step file) designed in MCAD software in Altium and place PCB footprints, and route it?
Defining the Board Shape from a 3D Body (https://www.altium.com/documentation/18.0/display/ADES/((Defining+the+Board+Shape))_AD)

This feature redefines the board shape based on a surface (face) of an imported 3D STEP model. It can be used to quickly create a complex board shape and helps integration between electronic and mechanical design areas. This is a two-stage process: first the STEP model is imported, then the required shape is selected from the STEP model. 

To do this:


Switch to View » 3D Layout Mode.
To import the STEP model (.step or .stp) place a 3D body (Place » 3D Body), then in the 3D Body mode of the Properties panel, select Generic in the 3D Model Type region. Select Embed Model or Link to Models in the Source region to import the required STEP model. Note that this STEP model can be deleted once the board shape has been redefined.
Select Design » Board Shape » Define from 3D body.
The Status bar will prompt you to Pick a 3D body. Click the imported 3D body to select it.
The cursor will change to a crosshair and the Status bar will prompt you to Choose Face. As you hover the cursor over each face, it will be outlined. Click to select the correct face.
The Board Outline Creation Successful dialog will appear, displaying options on how you want the imported 3D model positioned in relation to the newly-defined board shape. These can be ignored if you are planning on deleting the STEP model, otherwise configure the options as required.