"demote" domain to subdomain

[roderik]

We have a number of domains, that should be subdomains.
Is it possible to "demote" an existing domain to subdomain?
See situation below.
Any suggestions would be appreciated

Out of habit, we have created domains on our Plesk server as follows:
- ourdomain.com
- customer1.com
- customer1.ourdomain.com
- customer2.com
- customer2.ourdomain.com
...
So, each is a full-blown domain

We would like to rearrange this to:
+ ourdomain.com
|-- customer1 i.e.: subdomain customer1.ourdomain.com
`-- customer2
- customer1.com
- customer2.com
Of course, it is possible to do so manually, like this:
1: create a new subdomain "X",
2: transfer the content (e.g. with FTP) from customer2.ourdomain.com to X
3: remove domain customer2.ourdomain.com
4: rename X to customer2
I was hoping there is an easier way.