Yes and no.
If you have a manual portfolio that you want to convert to synchronized mode, this is not possible at the moment.
You need to create a new portfolio in synchronized mode and then delete the manual portfolio.
If you have a synchronized portfolio that you want to convert to manual mode, this is possible. You just need to remove the synchronization (but not the portfolio).
Follow the guide below for more details:
Delete a synchronization and its effects
How to delete a synchronized account and its effects