Yes! To create a new follow-up, go to http://muckrack.com/pitches and select an existing Pitch. When viewing your Pitch, select recipients to include in your follow-up and click Send follow-up.
Each follow-up will include the text (even if personalized) of your previously sent Pitch, and it will appear in the recipient’s email inbox as threaded like any other follow-up email. Sending a follow-up Pitch does count towards your email limits.