ok, here goes:
NOTICE:
This script is very basic and was done in one minute, and doesn't check if anything is present at the custom field 8, and will overwrite anything present there.
I could do a better version, but ATM this fullfills the objective.
I've tested it in my own contacts and does what it is expected to do

If it ruins your/anyones' contacts, I may not be held responsible, the script is distributed as is and you use it at your own risk.
Steps:
1. in applescript, paste the following code:
(*
sort contacts by first name
*)
tell application "Microsoft Entourage"
set allContacts to every contact
repeat with thisContact in allContacts
set myFirstName to first name of thisContact
set myLastName to last name of thisContact
set custom field eight of thisContact to myFirstName & " " & myLastName
end repeat
display dialog ("done!")
end tell
2. save it as script, you can name it: Sort contacts by firstName\cC
(the \cC part allows you to use a keyborad shortcut, in this case: Control - C )
Don't use an extension (.scpt)
3. place that script into the "Entourage Script Menu Items" folder, which resides inside the "Microsoft User Data" folder.
4. in Entourage, go to the contacts, and run the script (either by selecting it from the applescript menu, or via the shortcut); if you don't see the script in that menu, it means you didn't install it correctley, redo step 3 correctly!
5. once done, add the custom 8 field into the visible fields (this is done by doing a right-click or control-click and selecting the custom 8 from the dropdown).
Sort it by this custom 8 field.
6. that's it!