If it was a parked domain prior to the migrations it would now be a domain pointer, which is not a full domain in such way that it can have email accounts and such, the more typical use would have been for adding a common misspelling of a domain for instance.
You would need to remove it from domain pointers and then under domain management add it as a new domain. Do note though that some users has reported that messing with the files so for your own safety make sure to backup your site before making the changes.
The added domain will have its own root to which you need to copy the files as well, such as domains/<domainname>/public_html
If you look in filemanager, under domains, is there a folder with a name corresponding to the domain you want to add? If there is that is what is preventing the domain from being added, the solution is to rename the folder.
Once the domain is added you should be able to remove the new, empty, domain folder and change the name back for the one your files are in.
What is the domain you are having troubles adding?