Manage Computer drive letters are for local disks only. Parallels Shared Folders behave like network disks.
When you share a Mac folder and you want to assign to it a certain drive letter, then you should do the following:
1) Instead of sharing the Mac folder directly, share the parent of the folder. For example, if you want a drive letter for /Volumes/MyDisk/ParentFolder/TheFolder then share /Volumes/MyDisk/ParentFolder
2) In Windows go to My Computer and click Map network drive
3) Select the drive letter you want to assign. Enter the path to the folder you want to map to the drive letter. For this example you would enter \\psf\ParentFolder\TheFolder
4) Select "Reconnect at logon" so that the drive letter is mapped automatically when you restart the virtual machine.
The reason for sharing the ParentFolder is so that the path to the folder you want to share is as short as possible. Assigning drive letters to long paths has been known to not work properly.
The reason for not sharing TheFolder directly, is because it might end up being assigned with the first unused drive letter starting from Z instead of the drive letter you want.
Note that these mapped drive letters can't be used by processes that are running under a different user (users like SYSTEM, NETWORK SERVICE, LOCAL SERVICE).