While SpiceBoy has one idea - which works if you want want to share a directory of files.
Your original request wanted to amalgamate multiple directories under one 'share' first if I am not mistaken.
You could on the old DOS days use the JOIN command to map a drive letter to a directory (the reverse of a Subst), however that appears to have gone the way of all things with Win2K.
They have given a similar utility to the Unix symbolic link in the Windows2K resource kit - there is a program called linkd that allows you to create symbolic directory links on NTFS filesystems only [and only to other NTFS file systems].
So, if you want to place several drives/directories under 1 virtual share, so that you can collectively give this bundle of directories a drive letter then the linkd command will allow you to build a directory of links to the real directories.
You will need to obtain the Win2K resource kit - I think its free if you have a MSDN or Technet subscription.
You may also care to visit www.sysinternals.com to see if they have an equivalent bit of free software that works the same as linkd does.