FYI This is one of the reasons I gave up on the Samba/Posix zimlets. The Samba SID wouldn't show up for Samba Domains drop down list for the user accounts (new or old) and it wasn't an editable field. My SambaDomain and SID was listed under the Admin extension, just couldn't do it for the user.
The other issue was the fact that the required settings for Samba accounts in Zimbra aren't actually required settings for Samba (Profile paths). It would make more sense to follow the Must and May listings in the Samba.schema file for what is a required attribute and what isn't in order to make it more usable in future revisions. I still think the concept of having Samba in Zimbra is great as a whole, and this was certainly an excellent first release.