When you want to add more warriors and/or beasts to an existing legion, you can click on the “update” button at the bottom left side of a legion on the legion page. The system to update legions is very similar to creating legions.
You will have to pay the equivalent of the supplies needed (1 USD per warrior) for the remaining hunts of that legion, and also for the training fee (0.5 USD for each added warrior/beast). There are no discounts for supplies for 14 or 28 days when updating a legion. Remember that a legion can have maximum 10 beasts and that you need to add enough beasts to have sufficient capacity for the warriors you plan to add.
Here's an example:
If you have a legion with 5 remaining hunting supplies, and you add a new warrior, then you will need to pay 0.5 USD (training fee) + 5 USD (for supplies for 5 hunts) in $BLOODSTONE to add that warrior to your legion.
If you have a legion with supplies remaining for 6 hunts, and you add two new warriors and one beast, then you will need to pay 1.5 USD (0.5 USD x 3 training fee, because you are adding 2 warriors and 1 beast) + 12 USD (for supplies for 6 hunts x 2 warriors) in $BLOODSTONE to add those two warriors and one beast to your legion.