No. After conversion the NAS LVs will continue to use space in your unit(s). However, you won't be able to see them in the GUI, or in the Console, if you attempt to delete them.
Here the steps you need to take:
Back up the data on your NAS volumes first, if there is data you want to preserve.
Go to the Console to delete your LVs.
Select Remove LVs (Logical Volumes)
After saying 'OK' to the warning, and entering the Console password, you will see a list of the LVs in your system.
Select the LVs you want to remove by highlighting them with the arrow keys, and using your spacebar to select.
Exit the console when completed.
You may now start the conversion process to iSCSI.