I'm not really sure why you'd want to do this, but oh well. You need to understand how the games def files (in this case will be a .sii) is set up to provide different cabin options, and then edit it appropriately so you have the DAF cab and the MAN chassis together.
As you can't use mods in mutliplayer, you'd need to set up the truck in single player, save the game when it's ready and then open multiplayer up.
As an ETS2 MP Game Administrator, I would not recommend doing that. We've had users do that, which caused everyone within a 500 meter radius to crash, and we've had users that did that which did not cause anyone to crash. Overall, I wouldn't take the risk as if you are caught being the cause of someone's game crash you will get punished. Anyways, here's a thread which shows you the allowed mods in MP.