You are asking for existing features or in other words, you did not configure your RS-clients the right way!

Here a little how-to:

Assuming you have a central name-server in your local network or at least a hostfile in every client with an entry for each client you want to connect to.

The settings you must make at the RS-Clients:
--------------------------------------------------------------
Options -> Server:
[x] Manual Forwarded Port
[x] Dark Net: None

under the tab 'Network Configuration' enter the hostname you use in your network, the one your nameserver/hostfiles knows in 'Dynamiic DNS'

under the tab 'IP Service' disable (remove tick) 'Allow RetroShare to ask my ip to these websites:'

Before the first contact, you should assure that the Public Keys you exchange ends with the line:

--DYNDNS--"$yourhostname";

where "$yourhostname" must be resolvable by the other clients (as i assumed, by name-server or identical hostfile entries)

In addition, you can create your own bdboot.txt (I assume you know where to find that file) with the $ip $port pairs used in your network farm.

If you do everything like i described, you should be able to communicate internal only without connecting to the internet at all.

Chears,
Raphael Gradenwitz