Just make sure you start slowly, and gradually increase, your intake of fiber. I don’t think consuming other things would affect absorption, whether or not it is part of the shake, since it does really end up in the same place in your stomach and through your digestive system. However, there is a chance that certain consumed objects may change digestive workings. For instance, if you add some form of natural laxative that you didn’t know existed by mistake, bad things will happen. What powders/things are you wanting to add?