anyone has an idea on how to create a geoprocessing tool that selects a part of the map and turns it into a cad file?
You could probably use as a template like the clip and ship tool, try to look it up. You can extend that tool where after clipping data that is referenced in an mxd, you convert the data to a CAD file before spitting it out as a zipped content?
If you have access to ArcGIS Server so you can create your own go tool, I would suggest using a python script to select and loop thru the features you want, and the use the Export To CAD—Help | ArcGIS for Desktop tool to get it into Cade (untested by me...not at a machine where I can test this week)
Also an another sample may help you is this
Geoprocessing service example: Clip And Ship—Documentation | ArcGIS for Server
doing exactly what you want.
Download this from here
Retrieving data ...