Very easy, but a bit of work....
1) In the MediaPortal Setup - Tool, open the 'Television' section.
2) Now go to the 'TV-Channels' section, and create/edit one of your channels.
3) Go to the 'External' tab, and change the 'Type' to 'Recieve by external settop box'
4) Next select the 'Input via', and change it to 'CVBS#1'...
5) Now if you are going to change channels on an external device using a IR-blaster device, input the channel number into the 'Channel no on settop box'
If you have multiple tv-channels, you will have to follow the same steps for every channel.