Impact of late entries will vary by event.
You are welcome to apply for a late entry but the organiser reserves the right to decline it.
You are welcome to turn up on the day - if you can be added without significantly delaying the event then that will be fine.
In particular, if you substitute for a no show then that will be easy though you might have to fence under their name if updates cannot be done quickly.
If you don't fence then your entry fee will be refunded.