Seeding Data to a Destination Library (Tape or Disk), is definitely possible.
Obviously the first step will be physically moving the Tapes from Australia to Dallas
Once the tapes arrives in Dallas, all you have to do is to change Storage Policy Copy (where the Source Data is Tape) update Data Path to point to the Dallas MediaAgent and remove the Data Path for Aus.
Then for your Dallas Storage Policy Copy, you go into the Properties and select the "Source Copy" to point to the Tape Copy.
By doing the above, once you kick off a Aux Copy, since the Source Copy for Dallas is updated to point to the Tape Copy, it will read the data from Tape and write locally to the Dallas Library.
This will achieve your goal of seeding your data.