Yes it is possible. In DirectAdmin go to Domain Setup, Add Another Domain, fill in the domain information and press Create.
You will also have to change the nameservers of the domain to ns1.frih.net and ns2.frih.net. The website that you registered the domain at probably provides a control panel where you can change the nameservers of your domain.
After you have done this you will have to wait. It can take a a few hours (up to 24 hours is often mentioned but usually you don't have to wait as long as that).
Yes you can
1. Login to your Directadmin control panel
2. Select your subdomain in frihost (je yoursubdomain.frih.xxx)
3. goto Advanced Features - > Domain pointers
4. simply type your TLD
5. Change your TLD's nameserver to frihost server
hope it will help.