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.
**Reproductive technology and commerce**

Have the species forge trade agreements with land based settlements. Then formalize a 'marketplace' where excellent prices are offered to women who are prepared to sell viable, healthy eggs. Establish (or have land dwellers acting as licensed brokers) establish registered clinics where the trade is conducted under the supervision of local (land based) authorities who also get a % via the same tax rate they charge other trades. Then just use artificial insemination and artificial wombs to grow new colony members.