You can keep your domain registered with your current domain registrar or you can transfer it to HostMantis. If you keep it with your current registrar, you would just update the nameservers for the domain with your current registrar to the HostMantis nameservers provided to you upon signup.
If you choose to transfer your domain to HostMantis, your domain will automatically be set with the necessary HostMantis nameservers required for your hosting account.
Basically, your domain name will work with your HostMantis hosting account whichever method you choose. Whether or not you choose to transfer your domain name to HostMantis is a matter of personal preference.
For example, if you are getting a better price on domain registration than you feel you could get with HostMantis, you would choose to keep your domain with your current registrar and just update your domains nameservers. If you feel you would get better domain registration pricing from HostMantis than you are currently receiving, then you would choose to transfer your domain.
Updating your domains nameservers is usually the fastest way to get your domain name working with your new hosting account. Domain transfers can take up to 5 days to complete depending on the losing registrar.