Yes, you can add custom fields to the com_content (category? I assume you intend component?):
You can add a plugin as described here and then, simply via xml add the fields you desire, take a look here for the field types you can add:
Afterwards, you can add the fields in your template.
Hmmmm.... or do you intend you want to extend the categories component?
That is also possible, you just have to make a minor modification in the plugin i mentioned above:
Inside the plugins main php file, instead of writing:
That's it ;-)