Take a look at sp_repladdcolumn in Books Online.
* Warning: Books Online gives the wrong parameters for this sp - it misses one in the middle.
You can view the parameter list, with types, using the following query:
SELECT sc.Name AS [Parameter], st.Name AS [Type]
FROM master.dbo.syscolumns sc (NOLOCK)
INNER JOIN master.dbo.systypes st (NOLOCK) ON sc.xtype=st.xtype
WHERE sc.id = OBJECT_ID('master.dbo.sp_repladdcolumn')
ORDER BY sc.colid