A great question.
It seems that MyDMX will not let you do this.
There needs to be a feature added that allows you to "patch" one DMX channel to one or more other DMX channels, so when one DMX value is changed, this automatically changes the other channel(s).
As an example, if you had four 3 channel RGB Par cans, all physically addressed as #1, in the real world of course, this works and it is perfect.
However, for the sake of the visualisation, you would need to add all four fixtures to the stage, each with a unique address, or perhaps on different universes.
The software patches would then automatically follow whatever programming you applied to the first Par can, and the visualiser would do what you require.
This needs to be added as a feature request, as multiple fixtures on teh same DMX address is not entirely uncommon.