Why not? If you plan your work, and work your plan, the best solution is to drill a hole. Antenex makes a special 3/4 inch drill meant to install NMO mounts. It is just the ticket to drill through a firewall.
Contrary to popular belief, what you DO NOT want to do is use existing holes that already have wires or cables running though them.
If you want more information, visit my web site.
Alan, KØBG
www.k0bg.com