Introducing Interval Partitioning On An Existing Table For an Integer instead of Date

I'm trying to figure out the syntax for introducing interval partitioning to a table where the interval is an integer instead of a date.

I see lots of examples when it's a date: alter table x set interval (numtodsinterval('1', 'MONTH');

What I am trying to do is add interval partitioning to a table partitioned by range on a group number (integer) instead of a range of dates.

So, basically, I can create what I want by creating a brand new table with this clause:

TABLESPACE data

PARTITION BY RANGE(group_nbr) INTERVAL(1) STORE IN (data)

SUBPARTITION BY HASH (rec_ID) SUBPARTITIONS 16