Yes. To customize the subject line, go to the Newsletter that you would like to update and select Edit next to the current subject line. Once you click Edit, modify the text, and click Save when you are done.
You can also add a customized Introduction Message by clicking this box on the top of the single Newsletter draft preview screen. Personalizing each issue of the newsletter with an introduction or teaser "in your own words" can boost engagement and really give the newsletter a personal feel.