Before we start:
Modifying your system in this way may invalidate your warranty and should only be carried out if you're confident to do so. Lipo batteries, like others can be dangerous if mistreated so please read all safety information about your specific battery before use. I take no responsibility for you blowing up your transmitter, yourself, pets, etc...
I and several others have been using our TXs successfully on Lipo power for nearly two years now and enjoying weeks of flying without the need to recharge.
This short article will show how to convert your transmitter (TX) pack from NICAD NIMH packs to 3 cell 11.1v lipo. This article will cover converting a JR Macgregor (9X/II/DSX9) set so may differ slightly for Futaba / Hitec / Sanwa but please post if there are significant differences for clarification if unsure. Be aware that not all transmitters use 9.6v battery packs. DX6i for instance only uses 4 cells (6v) and in any case there is not really any suitable space for a reasonable size LiPo to be installed.
A fully charged standard 600mah 9.6v JR NICAD pack will reach around 11.5v, so it was felt that to give extra safety margin and to avoid blowing the replaceable fuse in the TX, that two diodes would be used to drop the voltage of the Lipo pack to around 11.1v fresh off charge.
Overlander Batteries currently sell a 11.1v 3cell "Hercules" pack that is designed to be a direct replacement for Futaba sets as it comes with a Futaba male plug already attached. This pack can also be used with JR sets but you will need to replace the Futaba connector with the suitable micro JST connector that JR use.
The pack shown in the following photos is no longer available from Overlander, but the principle is the same with any 3cell lipo that is of the right size to fit your transmitter's battery compartment (important).
Diodes can be obtained from Maplin
Other manufacturers are available who do advertise packs suitable for direct replacement with JR micro JST connectors already attached, but none come with voltage dropping diodes; A lot claim its not necessary to use any, as TX's should handle up to 12v current, however that's your call!