Absolutely!  This is how we track ecommerce activity, page titles, and any other customized website variables, without having to use page tags.  You can do this in one of two ways:
 

1. Add an additional term to the "HTTP Protocol" configuration that is being used by your SnifferReactor.  Use "response content" as the source and then define regular expression match and extraction criteria to pull out the bits you want to use to populate the field.  Take a look at the pre-defined "page-title" term for an example. For a step-by-step tutorial, please see the article "Using Pion's HTTP Protocol to extract additional content."
2. Use our Transformation Reactor to append an additional term to the event.  Create a new transformation and select a term that you want to populate with the content.  Use "Regex" for the transformation type, and then specify one or more regular expression transformations to use to extract the content. 

 
The former method is often best if a simple regular expression transformation is all you need, and you want to populate the new term for every event.  The latter method is often best if you have more advanced processing needs, or if you only want to populate the term for certain events. 
 
In both cases, we recommend creating a new vocabulary term to store the content you are extracting.  We recommend that you create own "vocabulary" to store these and other custom terms that are specific to your website.