You cannot add rate fees.
Conversion fees can be added as a regular fee on either the “buy” or “sell” subtype. Doing this would add either a fixed or percentage-based fee which the user would see the fee as a separate line item when confirming their exchange, and it would create a separate fee debit transaction for them. The credit portion of the fee transaction will go to a separate account stipulated by the admin when they set up the fee. Rehive recommends creating a standalone account called “fees” and using this as the fee operational account.
Padding via using the path functionality on a Conversion pair can also be used as a way of adding a “fee”. This type of fee would be built into the conversion rate and would not be displayed to the user as a separate fee, would not happen as a separate fee transaction, and would not be credited to any separate “fees” account.