Let’s make this clear, this is two different things. If you are beginner and not sure how to manage domain ownership, hopefully this simple article will give you some help.
1. Addon domain name is where you add domain name to your hosting account and host the new website in existing hosting plan and run it as a uniquely separate website. It’s not necessary registered via Justhost, it can be sitting at other domain registrar. What you need is to change the DNS to point to Justhost DNS name server and wait for domain propagation. Later add it to your cPanel account and manage the website from there. This mean your domain stay at existing registrar and the website is hosting with selected web hosting company.
2. Domain transfer is referring to change the registrar from the old registrar to the new winning registrar. The transfer process usually take 7 days to complete, you will need to unlock it from old registrar. Later initiate the transfer and enter the authorization key to approve it. Next, check the email confirmation for domain transfer and confirm it, and wait for the transfer process to be initiated. Once completed, the domain will be move to new winning registrar.
Either you are transferring domain to Justhost or not, you can host your website with them. By the way, their shared hosting account give you free domain name. If you require this Justhost domain transfer service, it is a free service. Note that registrar transfer are free and you can do it from your Justhost control panel and under domain transfer section as screenshot below