Does anyone have any information concerning adding onto a motor control center by connecting cables to the horizontal main bus? A breaker would be installed at the end of the MCC and it would be within 10'. I am actually hoping to find arguments against doing it. Thanks

I would begin by reading the 2002 NEC Part VIII. Motor Control Centers in Article 430.
tmon:

I can think of a few items that would lead me to believe that what you propose may be in violation of the rules in some parts of this place in the code, and in addition, 90.7 and maybe even 110.2 to give a few references for others to think about when responding to your question.

Does the cable ampacity equal or exceed that of the main bus? Are there provisions to terminate cables in the MCC or you making some makeshift connection?
Don(resqcapt19)

The cable would not be equal to the bus amperage. I could not find anything in the NEC stating that the tap conductors be equal to or exceed the conductors that are being tapped. The connections would be with mechanical lugs.

Sounds to me like you are adding a breaker because the MCC is tapped out. Are there provisions on the bus and literature from the manufacturer that will allow a connection to the bus? My understanding has always been that as soon as you start modifying equipment (in a way that it was not designed for) in the field you void the UL listing.
Depending on the buss rating and the actual load on the MCC you may get the manufacturer to allow this and re-certify the panel. Another way I have done this in the past is to install a "Top Hat" on the Mcc. (which is similar to a section of bus duct) that will allow you to attach an additional Mcc section next to or in the general vicinity of the existing Mcc. Make sure this is engineered, approved by the AHJ and who ever builds it can back it up with a UL listing.
Before you start. There are a few panel shops out there that can do this you just have to search. Depending on the size of your Mcc, length of new feeders etc. this can be a lower cost alternative than replacing the existing one.

What is the purpose of the extension.
Can you provide more detail?