You will need to add some custom script to the transformation and execute some C# code to send the email. You will also need to add some logic which will get the desired data and compose the email message.
IT might be better to create a custom web part for this. In this web part you can use the QueryRepeater control to get the data in the same way as the current web part + in your custom web part you will add the logic for the button click to get the data you want, compose and send the email.