Can I add a partition group to a partitioned table ?

Do you mean this?

Before:

SQL>select export_objects('','public.supplier_scd',false)
export_objects
CREATE TABLE public.supplier_scd
(
    spplr_key int NOT NULL DEFAULT hash(supplier_scd.spplr_id, supplier_scd.spplr_from_dt),
    spplr_id int NOT NULL,
    spplr_from_dt date NOT NULL,
    spplr_to_dt date NOT NULL,
    spplr_is_hist boolean NOT NULL,
    spplr_ins_ts timestamp NOT NULL,
    spplr_upd_ts timestamp NOT NULL,
    spplr_name varchar(30) NOT NULL,
    spplr_org varchar(30),
    CONSTRAINT d_spplr_scd_pk PRIMARY KEY (spplr_key) DISABLED
)
PARTITION BY (supplier_scd.spplr_from_dt);
[. . .]

Then:

ALTER TABLE public.supplier_scd
PARTITION BY spplr_from_dt
  GROUP BY CALENDAR_HIERARCHY_DAY(spplr_from_dt,2,2)
REORGANIZE;

Leading to:
SQL> SELECT EXPORT_OBJECTS('','public.spplr_scd',FALSE);
CREATE TABLE public.supplier_scd
(
spplr_key int NOT NULL DEFAULT hash(supplier_scd.spplr_id, supplier_scd.spplr_from_dt),
spplr_id int NOT NULL,
spplr_from_dt date NOT NULL,
spplr_to_dt date NOT NULL,
spplr_is_hist boolean NOT NULL,
spplr_ins_ts timestamp NOT NULL,
spplr_upd_ts timestamp NOT NULL,
spplr_name varchar(30) NOT NULL,
spplr_org varchar(30),
CONSTRAINT d_spplr_scd_pk PRIMARY KEY (spplr_key) DISABLED
)
PARTITION BY (supplier_scd.spplr_from_dt)
GROUP BY (
CASE
WHEN ("datediff"('year', supplier_scd.spplr_from_dt, ((now())::timestamptz(6))::date) >= 2)
THEN (date_trunc('year', supplier_scd.spplr_from_dt))::date
WHEN ("datediff"('month', supplier_scd.spplr_from_dt, ((now())::timestamptz(6))::date) >= 2)
THEN (date_trunc('month', supplier_scd.spplr_from_dt))::date
ELSE supplier_scd.spplr_from_dt
END
);
```

Correct. Current table does not have GROUP BY but want to add it, using same partitioning scheme.
I'm not sure if under the hood it'll 1) rebuild everything, 2) re-validate all rows, or 3) just apply the metadata change, possibly followed by mergeouts to combine into groups.