If the network is configured to allow this: yes.
To be more precise: port 5003 has to be free.
As a note (and an expansion for any of our lurkers who may be wondering how to configure this :) ), since you are accessing the database externally, the Server machine's router should be properly configured for port forwarding for 5003. A good reference site for anyone who needs to configure this is portforward.com
Port 5003 (TCP and UDP) is the port FileMaker uses for FileMaker Network Sharing so you'll definitely need that open on all machines/hardware. You can verify if Port 5003 is open by the following:
Win: command prompt > telnet 127.0.0.1 > 5003
Mac: utilities > network utility > port scan > localhost > 5003
Is this enough secure method (to access FM without VPN)?