how best to specify table constraints with "create table as <query>"

i want to recluster an immutable table without locking the table and
then add the table as a partition of a parent table. my plan was:

create table tbl_cpy as select * from tbl order by c1 asc;
alter table tbl_cpy add constraint c1 not null, c1>=lo c1<hi;
alter table tbl_parent attach partition tbl_cpy for values from (lo)
to (hi);

i want this to run quickly. so, i populate the table with a parallel
query (using create table as ...). i do NOT want attach partition to
rescan the table (which it does to validate the value range) so i have
to explicitly add constraints to tbl_cpy. but adding these constraints
itself causes a scan of the table.

so, i want to specify the constraints when the table is created. but
that seems not possible. if i first create an empty table with
constraints, then i cannot populate it with a parallel query (since
insert into is not parallel).

am i missing something? any ideas?

thanks,

kevin