I had this last week. I was told, categorically, on the phone and online chat that it was not possible to add my BA FF number to my booking (after selecting seats). Then I thought I'd try again as I'd read of customer services inconsistencies.

When the chap realised I was BA Gold he said that they can add BA FF numbers for anyone with OW status above Sapphire. Within 2 minutes he'd added my number and I was off the call.

So, make sure you mention your status level, if you have any.