Not sure if it is a UK thing, but MOST of our tournies only allowed rebuys once you have 50% or less of your starting chips.

Add on is usually full entry fee for starting chips.

However, I have organised many tournaments for our society and as the organiser you can do what you want!

The add on could be for half the buy in and double the starting stack! Anything that you think will make the tournament better for the players playing it.