There is this box
http://www.humaxdigital.com/uk/products/f2-foxt.asp that has an RF modulator (rather than just a passthrough), but it is £60!
---------- Post added at 22:36 ---------- Previous post was at 22:31 ----------
Apparently can do the following , so I assume you could tune each box to a seperate RF channel
- Modulator
RF/Loopthrough Input/ 75 D, IEC 169 - 2
Output Connector
Frequency 47 MHz ~ 862 MHz
Output Channel CH 21 - 69 for modulator
TV standard PAL B/G/I selectable by menu
Not quite sure how you would combine the signals, probably need several splitters in reverse