Where did the vmdk come from? You can use the datastore browser to copy. In the vSphere Client, Right Click on the datastore you want to copy to and select Browse datastore. Browse to the folder you want to copy the file to and use the upload icon to copy the file.
If the vmdk is from another VMware product you may need to use VMware converter to convert and copy the file.
My first thought is Veeam's free FastSCP product. VMware Converter might be another option, if you want to import an entire virtual machine.
Or even better, like Dave states - use the datastore browser!
Message was edited by: vmroyale