Could humans choose to establish fibre digesting colonies in our guts?
Probably not without some substantial evolution to change our physiology, depending on what you are wanting.  If you just want to digest fibre, that might be done, but if you wanted to see nutritional value out of it, its not clear that's going to be worth it..

Soluable fibre is currently used by the human body, so you are probably be asking about insoluble fibre.  

It might be possible to do a little of this experimentation - I'm not recommending this - but there would be several likely issues depending on your goals in such a project, which could vary.  


the human gut microenvironment probably will not accommodate the bacteria you need and they might not grow in significant quantity.
the human gut is not physiologically arranged for digesting such fibre - ungulates generally have four stomach chambers that they can use to store some of the hard for the longer time it takes to break the fibre down. 
the nutritional value of plant fibre is low XKCD has some numbers on this and a cow can eat 10kg of plant matter a day and in doing so they don't tend to move that much.  Humans have to eat a greater volume of vegetables to get the calories they need.  Is getting that last 2-5% or so of the energy from digesting fibre worth the trouble? Eat one more apple or a handful of grain.


Lastly, there would likely be some impact on human health - if you succeed then you would be modifying fibre's role in human diet as it stands - which is to help keep the intestines cleaned out.