You cannot move workspaces. However, you can create your target workspace at any level and then go to the files and folders in the original workspace and copy them. 

Be sure to copy everything with comments and with versions if you want to retain the versions.

Notes:

Workspaces are security containers and if they have a process attached, they must remain at the top level. 

But what you can do is create your target workspace at any level, and then go to the files and folders in the original workspace and copy them. 

Be sure to copy everything with comments and with versions if you want to retain the versions.

Once you have copied the folders and verified that they are all set properly, delete the originals and then empty your recycle bin. 

Note that if the original workspace is a process related workspace that is attached to an opportunity, you need to delete the original through the Web interface by deleting the opportunity so that all of the process, opportunity data and files/folders are cleaned up at the same time.

If you don't need the versions and comments, a very fast way to do this is to right click on the top level workspace (in the Platform Client) and choose the option to "Save As". 

Save the workspace and template and then go to the target and "Import" the workspace template with the Platform Client.