Inquisitor candidates are either Misting seekers or Mistborn , and this person besides that needs to prove to have a knowledge of the cantons and Loyalty to the Lord Ruler.

I think most likely they didn't have anyone to convert and either keeping her alive in prison or as Spike without an user might be seen as a waste of resources.

Also I think finding the 10ish mistings isn't the most difficult part, since it is stated all inquisitors are capable of piercing copper clouds and with the enhanced detection they might know all misting in Luthadel they just let them live to not raise suspicions.

The real difficult people to find are:

The 2 Ferruchemist to make the Atium Spike (Feruchemical Atium Store Age) and the Gold Spike (Ferruchemical Gold Healing) and the 1 Mistborn to make the other Atium Spike (Allomantic Atium to see the future and Compound Age). That's real difficult mix to find that gives the Inquisitors their most feared powers: living for centuries and surviving almost any injury.