How to add additional article into existing Publication.

Method 1: To add an article after a publication is created

1. On the Articles page of the Publication Properties – <Publication> dialog box, clear the Show only checked objects in the list check box. This allows you to see the unpublished objects in the publication database.
2. Select the check box next to each article you want to add.
3. Click OK.

https://technet.microsoft.com/en-us/library/ms152571(v=sql.90).aspx

https://www.mssqltips.com/sqlservertip/2502/limit-snapshot-size-when-adding-new-article-to-sql-server-replication/

Method 2:  To add an article after a publication is created through Replication Transact-SQL Programming.

EXEC sp_addarticle @publication = N’RMS_Publication’,
@article = N’lwcase’, @source_owner = N’dbo’, @source_object = N’lwcase’,
@type = N’logbased’, @description  = null, @creation_script = null,
@pre_creation_cmd = N’drop’,
@identityrangemanagementoption = N’manual’, @destination_table = N’lwcase’,
@destination_owner = N’dbo’, @vertical_partition = N’false’,
@ins_cmd = N’CALL sp_MSins_dbolwcase’, @del_cmd = N’CALL sp_MSdel_dbolwcase’, @upd_cmd = N’SCALL sp_MSupd_dbolwcase’
GO