HostPapa does support mailman. We also have a great script called phpList, which is the modern replacement for mailman for many people (cPanel > Softaculous > phpList).
For mailman, you can find documents to assist you here: https://www.gnu.org/software/mailman/mailman-admin/index.html
Also, from cPanel > Mailing Lists, you can click on “Modify”, next to the mailing list you created, for more!