You can't directly convert RMB deposits in your passbook into US dollar deposits. You can take out the RMB deposit first, convert it into US dollars, and then deposit it in US dollars.

There is no charge for converting RMB into US dollars, but the bank will sell you US dollars at a high "selling price", and now the interest rate of US dollar deposits is low, so after all this, you will not only lose money, but will lose money.