This is a very broad/vague question but generally speaking if you are doing PCB layout and moving it into mechanical design, the important information you need to know is:
-Board mounting holes
-Board components (or large component placement, depending on the precision level of your mechanical design)
-Component mounting positions
The actual board artwork can be useful for building the mechanical stack, I believe you can export an Eagle file to a DXF or another 2D drawing that will let you lay those objects out in 3D. You usually can also find IGES files for the components you are using on your PCB from a bunch of different suppliers rather than building anything from scratch. Also a cube is a good enough representation for components that you don't have specific details on, just bounding box dimensions.