Hi could anyone help me with this problem?

We have a LAN in our flat. There is one linux server with two network adpaters in it (one in one out. And three computers connected with bnc to this server. Now we would like to connect another computer to this LAN. Problem is that this computer is notebook and has only TP card. My question is should we buy a hub and change our LAN from bnc to TP (everyone then has to buy new cables) or could we only buy another network adapter for the server so there will be three network adapters(one in and two out)... I hope you get my point as I'm a little bit tired and my english is worse then obviously))

thank you very much sun7dots