T-SQL (SS2K8) :: Programmatically Generate A Number Sequence?

I'm trying to do a simple insert into a table, something like this:

insert into sometable (ID, somecolumn)
select 'Task-ID', somevalue from SomeOtherTable
where something = 'someothervalue'
(or something to that effect)

So, the SELECT would generate something that looks like this:

ID somecolumn
-- ----------
Task-ID somevalue1
Task-ID somevalue2
Task-ID somevalue3
(etc.)

Here's where my problem comes in: ID is a PK, and needs to be unique. What I need it to do is this:

ID somecolumn
-- ----------
Task-ID.1 somevalue1
Task-ID.2 somevalue2
Task-ID.3 somevalue3
(etc.)

What I don't know is, how do I programatically generate the number sequence? Note: I do not have admin rights to the table, i.e. I cannot just change a column to IDENTITY.

Also the 'Task-ID' must remain part of the ID; in other words, I can't just generate a GUID, and it needs to be easily identifiable.

What I'm hoping to do is rewrite my SQL like this:

insert into sometable (ID, somecolumn)
select 'Task-ID.' + (generated seq #), somevalue from SomeOtherTable
where something = 'someothervalue'

Is there an easy way to do this?