The best "substitution" is usually for the bard imprisoned by the white dragon in Book 2. I've already seen one swap using that (including having a love triangle as a result).
Another odd possibility could be the imprisoned girl in Artosia... assuming the character was in love with a man (the girl had been a man and switched genders using the magic of Artosia). In that case you have the oddity of having to cope with the fact your former lover is now the opposite gender. ^^;;
Once you get into Book 4, substitutions are exceedingly difficult.
Yeah I agree with Tangent. After the first three books there really is not a place you could fit an NPC like that in. I think the captive bard is the best bet. I did the same with a PC and his sister he did not know he had. It worked out really well. Just changed the names, I even toyed with having her up singing for the dragon when the party arrived but that did not happen.
You could put her in as a servant to a winter wolf in book 2. There is a part where one is trying to flee and you could put her there and he could save her. I honestly think book 2 is the best place. There are a lot of nice role playing encounters. at least in my groups experience. They went through the middle part of book 2 with hardly a combat.
Another option you could have Ringeirr with the love and he is either going to smuggle her in or smuggle her out of the Whitethrone. (I am assuming the love is female)
I hope these give you some ideas.
We did a pretty cool thing in the game that I'm running that you may consider. We established that one of the characters was a native of Heldren, and he took up adventuring after his wife died. She was a ranger of Ulfen decent, died about five years earlier, while carrying twins...