Unfortunately our new site doesn't have that capability at launch, but we will be adding it in the near future.
The alternative if you really wanted your ammo combined would be to sell and/or ship the existing inventory in the crate you don't want and then delete that crate. This will give you a credit for any ammo still in Queue. You could then add those funds to the new crate by subscribing to various calibers and buying up if available.
Another work around that could be an option if you wanted to combine crates for shipping purposes is to ship both crates in the system one after the other. This will then push to our system and we will see two shipments which we can then combine for you. If your combined crates would qualify for free shipping then we would refund both shipping charges, otherwise we would refund one shipping charge.