Hey Ali,
Welcome to the UK Community for QuickBooks Online, with anything; it can be confusing at first. Functionally speaking, you'd just add hours on through the process of running your payroll, increasing their wage. However, i have a thought:
Ideally, i think you'd be expected to roll back and set her up so you can pay her the pay owed in February, and then run March for March. I think i'm going to recommend speaking to a payroll accountant, or to HMRC to ask their advice on what should be done. I'm not any position of authority when it comes to your payroll.
My concern, is that if you add the hours and wages to February, she could be overcharged tax for the period; which we don't want. It might be worth checking, and coming back and letting me know what you're advised, we can then accurately point you in the right direction for using our system.
