Yes, the Giant Cod set is similar - that is a cheap 4-channel transmitter with a 6-channel receiver. The reason is that these items are built down to a price, and cheap standard parts are used where possible. It is probably cheaper to include an off-the-shelf 6 channel receiver than to go to the trouble of designing a special 4 or 5 channel receiver. They go for silly money - £25.82 for a 2.4Ghz Tx/Rx combo and £19.14 for a 35 Mhz pair!!!
These cheap sets are not designed to be upgraded, so there is no official facility to add a channel that I know of. However, the transmitter electronics probably include 6 channels, since that's probably what the basic transmitter chip provides. I know that the Giant Cod 4 channel transmitter has a number of unused sockets on its board, and if you plug a switch into one of them, with a little bit of soldering you get a 5-channel transmitter - here is a link to the forum discussion:
http://www.giantcod.co.uk/forum/viewtopic.php?f=15&t=3848 Maybe there is a Planet Users forum, and maybe they have found out how to add a channel...?