This is theoretically possible although I have not seen it or done it yet.
For short range you could go via serial wire communications - using ports on the Lego RCX.
For long range - you could:
- Try and see if the IR protocols are compatible(highly unlikely).
- Connect the RCX tower to the Serial Bus of the Cybot (bulky).
- Use a PC as an intermediary - with the Cybot Handset connected to the PC, and the RCX tower. This is the most likely connection to work.
If anyone has any further information on attempting this - please let the community here know.