Is the coriander in already? Are you planning on racking to secondary? You could always "dry hop" the batch in the secondary with some coriander. I don't know what the effect would be, but you'll impart some coriander taste with that approach anyway.
Ya, if you already added the whole seeds during your boil (at flameout or last 10min?) then you probably already got some of the coriander in there....might not want to add anymore, though I'm not sure really if you'll get much if you just add some to secondary
Taste it when you take a gravity sample in a couple of weeks. If you need more, add it at that time. Just make sure you sanitize it (steep in vodka, or in some 150 F water).