If you mean allowing incoming connections to the VM, yes, it can be done. First you will need to set your router to forward the appropriate port to the host machine (logically it's first, but for security reasons you should do this second to protect your host). Secondly (but do it first) you need to set the VM to use bridged networking and configure its OS to use a static IP and then use IPtables to forward all incoming connections on the port to the guest and all related outgoing to the host's internet connection.
There are lots of options for bridged networking, ranging from a direct connection to the host's internet connection (I wouldn't recommend that, though I did once use it to flash my router firmware from a Windows VM!)) to downloading bridge-utils and tunctl and creating a completely self-contained virtual network inside the host's kernel tun/tap device, to which only the desired traffic can be forwarded, before being forwarded on to the host (this is what I use).
There really is no substitute for reading up on IPtables, networking, Virtualbox networking options and the kernel's tun/tap interface. Enjoy learning all about it!
