If you are using the 433MHz band, it is mostly used for remote control applications up to a 100m. There are a number of companies making transmit/receive pairs for this sort of application, some have the built in capability to change the addresses in the encode/decode hardware to allow a number of transmit/receive pairs to operated in close proximity. If they don't, then a unique address can be sent as part of the data packet.
You would need to look at the range these devices are capable of, they often use a hookup wire antenna inside the housing, which is not optimal. To extend the range, you might getaway with a better antenna, there are chip antennas available. For better range at 433MHz, better matching, or a dipole, whip, or yagi may be needed, but they may be too large for your application. There will be a maximum allowable power at the antenna permitted by law, you will need to verify that for your country.
The RFID reader presumably just puts out a code in a serial fashion. If the reader allows customization, you may be able to program it with your own code, otherwise you would need an arduino to process the serial data and send it to your transmitter. The data packet would contain the code from the reader, along with a unique address if needed, and this would be transmitted to the receiver a Km away. The raw data would be processed by another arduino. If the address in the data packet is assigned to that receiver, it will then pass on the code from the other end to be used how you see fit.
There are likely to be all sorts of other issues as well.
Before you start, you need to verify that your transmit/receive pair will operate over the maximum distance you require under the worst case use, within the maximum power limits permitted where the equipment will be used. Additionally, a number of units operating close by may cause blocking, where a transmitter that is not associated with the the communicating pair may overload the receiver of the associated pair, and the signal will not get through. This may mean sending multiple times, or using a receive acknowledge scheme, which requires a transmit/receive pair at either end of the link. This presumes that there is dead time from the blocking device. This increases the reliability, and may be needed where the signal is marginal in any case. Directional antenna are an advantage here as well, but they may get too cumbersome, expensive, and are may be difficult to align.
Without knowing a lot more about your intentions, it is difficult to suggest much more.
Cheers,
Richard
