Well i got in touch with Onkyo and here's what they came back with.
"The remote control of the receiver TX-SR 674 is pre programmed.
That means the commands for the different devices are stored in the remote
To activate this stored controls you have to give in a specific code
Unfortunately it's not possible to expand or to learn new commands into
the memory of the remote control.
Therefore we can not give an new code."
I'd guess this is the same for a lot of products.
Thanks for the help Andrew.