You cannot add dynamic padding to custom rate pairs and it would not be advisable to do so. Adding padding to a custom rate pair will mean that any rate displayed in the app is incorrect. A custom rate pair should always represent the raw exchange rate.
If you want to charge users an additional amount when making conversion transactions between currencies you can do this either by adding a fee on the “buy” or “sell” subtype in the currency you would like to charge the fee in, or you can set a path that would add a percentage to the rate when creating your conversion pair. By adding the padding in the conversion pair instead of the rate pair, you ensure all displayed conversion rates are correct and you will still be able to charge an additional amount to users when they make conversion transactions. This would work more like a hidden fee or a spread to hedge against currency volatility.