Is there a process to use an already patented idea that I can use along with my idea? Also if not, how can I protect my idea if I approach the original company to pitch it.....What can I do to protect my idea and keep them from stealing it when I approach them with it?
You can obtain a patent on an improvement to an existing technology. This is done all the time. One thing to remember, just because you may obtain a patent on your improvement, doesn't mean you avoid having to obtain a license from other patents already in force.
A common strategy is to file a patent application before approaching other companies about licensing opportunities. In the US there is something called a provisional patent application which is often used for this purpose. Patents themselves take significant time and money to obtain, but at least the application provides some protection from someone stealing your idea.
Although it is possible to file for a patent application on your own, I highly recommend you work with a patent attorney or agent. A poorly constructed patent may provide little real protection. If you do some of the work up front, you can make the process of working with a patent lawyer more efficient. Please see this answer for some suggestions. The attorney may also be able to provide you with guidance about contacting the firm in the safest manner.