If you try long-pressing on any books that you have on your device, it will allow you to add it to a collection, but this warning pops up, saying this will not sync the item to cloud.
According to André Klein September 27, 2015
trying to get Kindle PC to accept custom dictionaries ultimately proved futile
but he does go on to mention:
binarycrafts from Romania have found a way to circumvent Kindle Cloud reader’s blocking of text copying by use of a simple bookmarklet.
The code for this is copied below:
If a matter of storage space on some local device, the following from sharechair (dated January 21, 2013) may be of interest:
removing the books from your Kindle or Kindle App does not ‘delete’ them from your life. It only takes the book off of the device .. but it leaves it in the Cloud, so that you can put it back on your device whenever you choose.