Feasibility of Partitioning existing tables with huge data in Oracle

where old table will be static and new table will be partitioned, and select will be a union of the 2 tables as required

This is really easy to do in an instant without copying anything or requiring any additional space, and you do not need to use UNION or rewrite anything at all, because you can do this with 1 table.

Let's say your table is this:

create table t1 (col1 date, col2 number);

Then you create the partitioned version of this table with 1 partition where you can store all your existing data:

create table t1_exch (col1 date, col2 number)
partition by range (col1)
interval (numtodsinterval(1, 'day'))
(
  partition p1 values less than (date'2017-03-11')
);

Then you exchange your table with the partition:

alter table t1_exch exchange partition p1 with table t1;

Finally drop the old table, and rename the new table to use the name of the old table:

drop table t1 purge;
alter table t1_exch rename to t1;

Data inserted after '2017-03-11' will be automatically partitioned and you can leave your old data as it is, or split into partitions if you want.

Partitioning data is easy, the cumbersome part may be dealing with the constraints and indexes that need to be global. You may need to change your PK and Unique constraints (and indexes), as you can not have such indexes locally partitioned without the partitioning key included in them. Also because of dropping the old table and having a new one in its place, you need to sort out the FK constraints pointing to this table.

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:

1. you can add a date column (say sql_date) to that large table using default sysdate. Thus only the new records will store current date.
2. Create index on sql_date.
3. Update sql_date batch by batch.
4. create a partitioned table with some other name.
5. copy the data to partitioned table using CTAS batch by batch.

Once the data is 95% copied. Take a down time.

6. Copy the remaining data and
7. Rename the tables (swap the names)
8. Add the grants
9. After through testing drop the old table

Only downside to this is extra space equal to the size of your table(1TB).