Please be aware that changes to your nameservers can take 24-48 hours to propagate globally and improper settings can take your site offline for extended periods of time. When in doubt, contact Reclaim Hosting support in advance of these changes to make sure they are being put in properly.
At this point you should add an addon domain to your existing account if this is an additional domain (you can ignore the first part of the guide about buying an additional domain from us):
After that you should be able to use it with your existing account. If your existing account is using the domain you want to addon as the root account, you can just add it as an addon domain and it will resolve to public_html.
Let me know if you need any specific help with this, and I would be happy to help.