An opinion from another site:
This person hopes that kicking can be added to boxing without changing it in any way. I can see why they hope this- it would be much easier if we could just pick the bits from various systems that we like and add them together unaltered to make a coherent whole. Unfortunately when we do this, what we end up with is often uncomplimentary bits and pieces conflicting with each other, amounting to an incoherent mess, rather than a functional system.And I can add low-line kicks to my boxing structure and it works just fine. Panantukan does that. You can have kicking equivalents of a jab, cross, and hook, etc. You can even mix them into combinations with the punches.....jab punch, cross kick, hook kick....jab kick, hook kick, upper cut punch....etc. All still using the basic boxing "engine" or biomechanics and boxing strategy.
Boxing footwork, biomechanics, strategy and power generation are all predicated on the assumption that kicking, kneeing, and clinch grappling are not possible. As a result, due to weighting, stepping patterns, strategy it is uniquely vulnerable to kicking, kneeing and clinch grappling. This can be seen when fights between boxers without cross training and Muay Thai exponents are examined.
In fact only certain limited elements of boxing work where kicks, knees and clinch grappling are possible. The full system is not viable. An example of a stance half way between a MT stance (very safe and defensive against kicks and clinch grappling, weak for punching) and boxing (dangerously vulnerable to kicks, knees and clinch grappling, better for punching) is the standard knockdown karate stance.
I hope this thread gives some guidance to beginners lacking knowledge in these basics, and provides some pointers in terms of modifying boxing for fighting where kicks , knees and clinch grappling are possible.
Here is what typically happens when a boxer tries to fight in a venue allowing kicks: