You can create APR increases in the system to increase tariffs by a set percentage.
In the Control Panel > Retail Tariffs > APR adjustments
You set what type of what you want the APR to effect the date you want the increase to come into effect and the percentage (if you are wanting to add the APR to just a handful of accounts you should enter 0 and set the value later on the account).
Once created you can then add a target to the APR
Which can be global for all account, a particular office, or particular framework.
If you would like the APR to only effect a few accounts you can set the percentage as 0 and a target of global, in an account under edit details > APR Increase you can set a bespoke amount