Try the following
1) drop the constraint on the table
2) add the column
3) add data in this column
4) make a new primary key with the 5 fields
To get a list of constraints on the table, try
select OBJECT_NAME(OBJECT_ID) AS constrnt
FROM sys.objects
where OBJECT_NAME(parent_object_id)='AccountMappingMaster'
and type_desc LIKE '%CONSTRAINT'