To expand on Ron’s reply a bit. One of the main benefits to having a separation between Things and Items is that you can build your Items independently of the Things.
So build your Items as if you actually had the device. Just leave them unlinked. Build your sitemap or Rules as you need them. Then you can command or update the Item using:
another Rule you use for testing
the sitemap
the Karaf console
the REST API Docs
When you get the device and get it working with OH, then all you need to do is link the Channel on the Thing to the Item and you are done.