Question: The male klepton is a unique humanoid. It has no females, and reproduces by mating with other humanoid females, who bear children that are clones of the klepton father. No females are born, and all offspring belong to the klepton species. While a settlement of these beings could simply obtain the necessary mates through kidnapping, this isn't exactly sustainable; raiding cities to enslave their women (or anyone else) isn't exactly good for diplomacy

How could this humanoid maintain a settlement, where most of the population is of the klepton species and the children end up with their fathers, without the need for any kidnapping or sex-slavery?

Answer: 
Collecting baby girls

In many parts of the world, particularly in historical situations, you can obtain all the girls you want by picking them up where their parents abandoned them. It was notoriously how brothels found their staff in ancient Rome.

Kleptons gather the girls, raise them in their families, and marry them off as if they were their own daughters.
The humanoid females don't need to be married forever; they can simply stay until the child(ren) they bore are fully developed, then they can freely go back to their places of origin to seek new mates if they so please (if they want to ensure their own DNA gets passed down).

If this is a klepton species that's closely related to other humanoid species, I think that the more likely mechanism isn't the children being clones of the males; rather, during the process of gametogenesis, the klepton species' gonads discard the portion of the genome inherited from their mothers. A similar mechanism usually goes into play among the most well-known examples of klepton species. This ensures enough genetic variation to keep the species healthy while also maintaining its distinctness.