You can create a new primary key field, defined as an auto enter serial number, then use the "Replace" (update serial numbers) in the filed to fill it with serial numbers.
Have a look at the Replace Field Contents menu command in the Records menu - you can find the records with no primary key and use this command to populate them all at once. You can nominate a starting value as well. Just make sure you won't end up with any duplicates with existing records.
tthanks. Very helpful