The function of any bolt-on is to provide an additional allowance or allowances. They act prospectively, not retrospectively.
If any user has incurred additional charges, adding a bolt-on will not cancel those charges.
The best way to manage costs is to add a Spend Control¹. From the dashboard of your my.plan account, go to:
CONTROLS > SPEND CONTROLS > DATA LIMITS/CALLS LIMITS/TEXT LIMITS > [select user] > SET UP
Alternatively, set an Ofcom Total Spend Cap². Go to:
CONTROLS > SPEND CONTROLS > OFCOM TOTAL SPEND CAP > [select user] > SET UP
¹ available to Gold & Platinum account users and Essential account users via an upgrade
² available to new customers from 1st October 2018 and existing customer who renew from the same date