You cannot synchronize existing AD passwords to SAP and you cannot redirect the SAPGui logon screen password to probe AD passwords. Not possible.
What you can however do is "hook" the password in cleartext before (re)setting it in AD and then send it to SAP as an initial or even productive password (eg. when the user resets their own password in IDM).
But none of these are SSO and LDAP sync cannot sync the password. Simply not supported on the SAP side (which triggers the process) and also not on AD side (to return cleartext passwords).
Not possible...
Cheers,
Julius
PS: your duplicated question is locked.