ME;Have never found out how to do it, at no extra cost at time of original long-haul booking.
bear in mind that different ticketing rules apply and adding the stopover might increase/decrease the overall cost.
As always, if you can be flexible in your dates and timing, you can get cheaper.
Did ring KLM a while back , but got tired of hanging on.
I'm very flexible, time wise, less so money wise.
Mostly it's travelling KLM to South America, Central America and Caribbean, out of Dublin. I really like Amsterdam and Schiphol is not a magical mystery tour like LHR.