You can use a synced table with the Portal. However, you cannot edit the synced fields. Only the added fields can be edited by the portal user.
On the legacy site app.miniextensions.com
It is not possible to set up the portal extension on a synced table. It has to be set up on the main original table. If setting up the portal on the original table is not possible in your case, another option would be to use our Search Page extension. That extension can be set up on the synced table. It has different features though, but it might work just as well in your case.