Filling Missing Primary Key Values

   I've a table (from a client, not created here) with a column that should
be the primary key, but not all rows have a value for this attribute. The
column format is VARCHAR(12) and has a variety of values, such as 96-A000672
and 9612-0881 (probably assigned by different analytical laboratories).

   A simple sequence of numbers would do the job of replacing NULL values.
What is the most parsimonious way to replace NULLs with unique values for
this column? I also need to add such values for a new set of data that I'm
in the process of translating from spreadsheet format to the table
structure.

Rich