How could I appear as a specific IP address to access

using a vpn you can access your network if you're not in the office, as if it were a computer over the local network, with a IP of this range, of course

You can appear to be a specific IP by using a technic called IP-Spoofing the issue with this approach is, that you can send but not receive. Since the IP packets originate from another IP Adress the host will send those packages to another Peer that has the spoofed IP Adress.

The other way to do this is using the Quantum Technology (FoxAcid server) to do a man in the middle attack on the host. This technology is only available to large intelligence agencies like the NSA with partnerships to national Telcos.

I don't know why your IT Department relies on IP-Adresses tough, because it simply isn't feasible for people that want to work out of office. Even the smallest company has a VPN of some sort nowadays. Almost all consumer grade routers support VPNs and professional routers did so for many, many years. Its easy to set up and very secure if implemented right.