You don't - in any Activity you can customise ANY commands from ANY of its Devices that you're ever likely to use to any convenient hard or virtual button.
It's an example of what I said - the Devices area of the software is for use when setting things up, after that it will have very little use, and the Devices area on the remote should have no use at all.

Although you can map any participating device commands to an activity, they eventually clutter the activity(ies). The sound and picture buttons (on some models) provide a direct short cut to the 2 mostly used devices. Although you can achieve the same thing in device mode, editing their button mapping is a real pain compared to that of the sound and picture mode. Unlike the device mode, which comes pre-mapped with the entire device command dataset, the simple default mapping for the sound and picture mode, which is very similar to that of the activity, makes editing a lot easier.

While I never had to use the picture and sound buttons they do come in handy for these types of situations.

I wish they would simply provide a button to flip between 2 hard button configurations for each activity. That would accomplish the same thing but be more generic.

I've no experience of the models with "picture" and "sound" buttons but can see their advantage.
Even my more complex Activities include all the buttons I'll ever need without any clutter at all.