Feasibility of Partitioning existing tables with huge data in Oracle
Oracle Database provides a mechanism to make table structure modifications without significantly affecting the availability of the table. The mechanism is called online table redefinition. Redefining tables online provides a substantial increase in availability compared to traditional methods of redefining tables.


You can redefine the table online and it will be accessible for DML. The table will be locked in an exclusive mode for a negligible period of time, where the size of the table doesn't matter. 

The downside is you have to have extra space equal to the size of your table(1TB). 

Update:


  Online Conversion of a Nonpartitioned Table to a Partitioned Table
  
  Nonpartitioned tables can be converted to partitioned tables online. Indexes are maintained as part of this operation and can be partitioned as well. The conversion has no impact on the ongoing DML operations.


In Oracle 12c Release 2, you can convert your non-partitioned table to partitioned table using ALTER TABLE statement. But, as per your tagging, you are in 11g R2 and you don't have this option.

Documentation:
 Redefining Tables Online
Oracle 12.2: Online Conversion of a Nonpartitioned Table to a Partitioned Table