Copy FileGroup to Another FileGroup
You can accomplish this a couple of different ways, but probably the best way would be with a table partition.  First you create your filegroups in the database and add at least one file to each.  Make your existing table a partitioned table by following the wizard (there's more here than meets the eye.  You will need to read up on table partitioning).  Create a staging table that matches the structure of your existing table, but without any indexes or constraints or anything. Load your 1TB of data into the staging table. Once it is all loaded, then add your indexes and contraints and stuff.  Then, you ALTER TABLE stagingtable SWITCH TO existingtable.

Since you didn't even know that table partitioning existed, you'll want to educate yourself on what a table partition is before you up, jump and implement in production:
http://www.brentozar.com/sql/table-partitioning-resources/

Not only should you read up on best practices, but you will want to try it in your test environment first:  http://msdn.microsoft.com/en-us/library/cc966380.aspx