By hand, your best bet would be to use the multiple select feature. Go through and select the text you want, holding down the command key. This should select each piece separately Then, once all your text is selected, apply the style using the styles tray.
You might prefer some automation. You could try using AppleScript to copy your document to a new correctly styled document. Here's one example I found. Otherwise, the underlying format to Pages documents is XML. You may be able to use an XML transformation language like XSLT to add in the appropriate style declaration where it belongs. Or you could use another language like Python/Perl with some carefully crafted regexes. Be careful though, messing with the underlying XML is considered dangerous.
You may also want to look out for a more systematic error. It sounds like you have content already, and you want to apply a certain kind of formatting for it. Pages may not be the best tool for what you want to do. Instead consider other options for storing and manipulating the data you have. XML or another hierarchical format would work best. That way you can store your content demarcated for each piece: <Title> <Comment> etc. You can then use something to translate this content to a desired style, if you went with XML, you'd use XSLT. Using XSLT, you could translate it to RTF, HTML, or just about anything. If you went with HTML, for instance, you could easily apply a CSS to manage your styles.
