You can do what you want by selecting the block in your drawing.
Copy the selection. 'Edit->Copy'
Open a new drawing.
Paste the block into the new drawing, and move it's reference point to the red cross.
Explode the block (by selecting it then 'Modify->Explode')
Then save it to your library as whatever name you wish..
Make sure that it's reference point is on the red cross before saving it, otherwise it will land in an unexpected part of your drawing when you use the library item later.