Use this in a script:
reg add "HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders" /v "Personal" /t REG_SZ /d "<path to share>\%USERNAME%" /f
reg add "HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders" /v "{f42ee2d3-909f-4907-8871-4c22fc0bf756}" /t REG_EXPAND_SZ /d "<path to share>\%USERNAME%" /f
reg add "HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders" /v "Personal" /t REG_EXPAND_SZ /d "<path to share>\%USERNAME%" /f