you can easily add new column in table as identity

alter table TableName

Add ColumnName int identity(1,1)

but, to alter exisiting column and making it as identity is not straightforward... since, u might have some data in existing column which does not follow identity....

this article explains the details

http://blog.sqlauthority.com/2009/05/03/sql-server-add-or-remove-identity-property-on-column/

hope this helps...