There's a total of seven base and worthless miscreants whom you can imprison for later ransom:
Nyrid, in Copperlane. He's a wizard who has disguised himself as a Svef dealer, whom you'll encounter in the course of the His Old Self side quest, picked up from Kaenra at the Goose and Fox Inn.
Eorn, also in Defiance Bay, you'll cross paths with him while pursuing the main quest through the Catacombs.
Naelde, is the leader of a group of Leaden Key assassins that will ambush you in Act 3. You can choose to spare her and take her hostage.
Gramrfel the Wayfarer, whom you've already found, will appear at Magran's Fork with his band of ne'er-do-wells after you've been to Defiance Bay for the first time.
Aefre is harassing the fine ladies of the Salty Mast in Ondra's Gift. If you go there and pick up the side quest Supply and Demand, they'll send you to deal with her, and you can offer to let her rot in the dungeons, rather than putting her to death in the streets of Defiance Bay.
Esmar is a Mercenary Captain whom you'll encounter during the side quest At the Mercy of the Tribes, picked up at the Celestial Sapling Inn in Twin Elms; as one potential resolution for the quest, you can imprison him as a guarantee for the safety of his men.
Kestorik is a scout who you'll find in the lower levels of the Endless Paths of Od Nua.
Do keep in mind however, when imprisoning any of these individuals, that doing so means that you're trading any loot they might be carrying on their persons for the chance at a ransom payoff later - if you're Security at your keep is high enough to prevent an escape before it comes in. And some of them are carrying some very nice loot!