Please clarify
The Devices are imported through a Active Directory Discovery
So you have configured Active Directory System Discovery and this has found several devices which exist in AD. There will be no MAC addresses associated with these devices as AD does not have this information.
In fact, those PCs should be installed through SCCM.
What does this mean? Are these not already working machines? Do you mean that you need to install the SCCM agent on these machines. You should be able to do that - Right click > Install client
After the agent is installed the MAC address will now be known.
But the Deployment won't work because there is no MAC Address given in the Device Entry in SCCM
Don't quite understand this.
But the Deployment won't work because there is no MAC Address given in the Device Entry in SCCM and i see no way to add one
As Torsten said you shouldn't need to do this. However if you insist open SCCM Console and click on
Assets and Compliance, click on Devices, in the ribbon above select
Import Computer Information