The simplest way to accomplish this is by using a standard multiplexer or quad connected to the looping outputs of the DVR as shown in Figure 1. This will give the guard the ability to select which images are displayed on his monitor without affecting the DVR and without installing a network.
If the DVR is network-capable it is normally a simple procedure to add it to an existing LAN. You will have to get an RJ-45 network jack, a patch cable, a static IP address, Subnet mask, possibly a default gateway from the IT manager, and possibly remote viewing software from the DVR manufacturer.