RS-232 is point to point, so you cannot do anything else other than bring all the devices into your server on separate lines. You could use some sort of concentrator (either pure RS-232, or RS-232/USB converter), however the physical link problem you are dealing with was solved a long time ago in industrial processes and is RS-485. This allows point to point and multi-drop network topologies for the physical layer and has a lot longer range than RS232. In order to communicate with the devices you would have to run some sort of intelligent protocol that addresses each node. (Typically something like Modbus would be used). Thus your terminal equipment would then have to become intelligent enough to respond to the network addressing. This would also fulfil your requirement of adding a prefix to the data stream - it would end up being the network address. But the downside is that your software implementation on the server side also has to become more complex in order to speak the same protocol and allow for things like network configuration.
IMHO if you only have a small number of devices to connect to, you are probably better off using a multi-port serial card in your server and running all the devices to a central location. However this is predicated on how much cabling you will have to run in parallel. It could be that the overall cabling cost could push you into the realms of an intelligently designed network system (pun intended).
Note that given your application is a parking garage, I imagine that your cabling lengths are going to easily exceed RS-232 maximum distances (50 feet). In which case you will also have to be implementing some sort of media converter on each end of the cable so you end up with something like:
RFID Device ==> RS232/RS485 ====(long cable)====> RS485/RS232 ===> Serial port ===> Server
(Note that you could also replace RS485 with your fav physical layer - including fiber)
Edit Not that I have used them, but you can get RS232/Ethernet extenders that could also work in place of RS485. Cat5 is a well known cabling system (so should be cheaper to manage) and Ethernet has a longer range that RS232. But once you exceed the Cat5 range (100m, about 328 feet) you will need to have some sort of media converter as well.