Well, if you really needed to, yes you can .. sort of. RJ 12 has 6 pins where as RJ45 has 8, but thats really not the problem since normal ethernet 10/100 only uses 4 wires or 2 pairs... the problem is fitting the rj12 into a rj45 socket! The pins will not line up correctly as the space between them are not the same on both connectors.
The reason I want to attach RJ11 is that I want to replace the plain flat telephone wire with a UTP wire because it is getting damp (ygrasia). I wish to use the UTP wire not for Ethernet but for ISDN and Telephony.
All I wanted to know was is it was physically possible. I assume that it is.