If you have ms sql srvr mgmt studio you can do it. You just have to do it manually, not in script or command form.
connect to your db
show the table/column names in object explorer window
right click on column name
select modify
in column properties tab
alter "identity specification"/"is identity" to YES
set increment/seed properties as desired
You can't alter the existing columns for identity through query
You have 2 options,
1. Create a new table with identity & drop the existing table
2. Create a new column with identity & drop the existing column
check this
http://social.msdn.microsoft.com/forums/en-US/transactsql/thread/04d69ee6-d4f5-4f8f-a115-d89f7bcbc032[
^]