Dear OliP,
in a vSphere environment (can't tell if it's the same in standalone mode) you can edit the newly created VM when starting the convert if you are quick enough and paste the MAC before it gets started (you have at least 10s to do so).
Now the really tricky part is if your hosting environment assign you a Gateway which is not in the assigned IP subnet..
My solution:
- extract /boot/filesys.gz from C:\Program Files\VMware\VMware vCenter Converter Standalone\converter-helper-vm.iso using i.e MagicIso on Windows
- gunzip and mout -o loop filesys /mnt on your favourite linux-based OS
- edit /mnt/usr/lib/vmware-converter/configureHelperNetwork.sh by adding after "/etc/rc.d/init.d/network start" the following lines:
route add $GATEWAY dev eth0
route add default gw $GATEWAY
then undo all the previous steps (umount, gzip, reinject the file using MagicIso).
Now when the converter helper VM starts, it properly adds the gateway and it works!
hope I could help,
JM