To combine multiple barrels or to combine barrel groups in to a single barrel group you can use the Multi transfer (many-to-one) operation.
You will firstly need to select all of barrels in the source vessels on the left hand side of the operation. Click Add Line to include more vessels.
If you select a barrel group and only want to use some of the barrels from that group, click the Barrel group list icon to the right hand side of the vessel, outlined in the screenshot above.
You will then need to mark the Fill state to Empty on the barrels required and double check the Out Volume matches what you expect.
When you've completed the source vessels you can then create a new barrel group as the Destination Vessel. Click New to the right of the Vessel field to create a barrel group. You can then add all of the barrels you need in this group. We've got some instructions on creating a barrel group to guide you through.
Once the barrel group is configured enter in your transfer in volume and save the operation.