If you simply execute a DDL, you will break the environment. You need to quiesce the group, use alter_master_repobject to update the table's definition, and regenerate replication support. It can be a bit complicated .
Oracle Certified Master DBA
It§s depend if you use mutlimaster or master-snapshot replication.
In mutlimaster you need to use DBMS_REPCAT.EXECUTE_DDL, but read more in doc how to propagate DDL to others sites.
In master-snapshot I use those steps:
On master site:
1/ suspend master group
2/ remove table from master group
3/ alter table
4/ add table to master group
5/ resume master group
On snapshot site:
1/ drop snapshot
2/ create snapshot
3/ add snapshot to refresh group