From CATIA to ldraw :
Save you CATIA part to STLl and use stl2dat to get a DAT part.
Be aware that such part will not be ldraw standards compatible and will need to be reworked. If it is only for you own use, then it would be fine.
From ldraw to CATIA :
Use ldview to save you DAT part to STL, import you STL file into CATIA. Then you will need to rework the mesh, close holes and you should be able to solidify it to get a CATIA part.
Save you CATIA part to STLl and use stl2dat to get a DAT part.
Be aware that such part will not be ldraw standards compatible and will need to be reworked. If it is only for you own use, then it would be fine.
From ldraw to CATIA :
Use ldview to save you DAT part to STL, import you STL file into CATIA. Then you will need to rework the mesh, close holes and you should be able to solidify it to get a CATIA part.