Edit: Add hint for setting up virtual device:
First set up an additional device e.g.:
ifconfig eth0:1 up
The you might additionally add an IP to it. e.g.:
ifconfig eth0:1 10.0.0.20 broadcast 10.255.255.255 netmask 255.255.255.255
If you really want to, you can also alter the MAC-address:
ip link set $DEVICE address $MAC
Note that the second bit of the first byte has to be set to signal a locally administered address (LAA) - which says that this MAC-address is only locally valid (e.g. within your enterprise network) and might not be unique world wide (so you can assign your own MAC-addresses without the need to register them officially).