You can add new domain to multiple domain Linux Shared Hosting plan through "Addon Domain" feature of cPanel. You will just need to login to cPanel account, click on "Addon domain" icon to add a new domain. You can refer following knowledgebase article for step by step guidance:
https://manage.accuwebhosting.com/knowledgebase/1993/How-to-create-an-addon-domain-in-cPanel.html