How do I use the bone tool on a symbol that has 28 frames of animation already?

Hi mate,

Just forget about the bone tool. It will only give you grief and waste countless hours of your life if you decide to persist for a while...

There are other systems that should give you better control such as the built-in Layer Parenting or the Smart Magnet Rigs. 

It's usually important to plan and rig before you start animating, but it is not impossible to do it afterwards. 

Unfortunately with these things you usually learn from making mistakes, so I wish you good luck!