KLM only offers codeshare flights on itineraries that also include its own flights. You cannot get them to issue a ticket including only Transavia flights (or any other codeshare operator, with the exception of AF).

So, if your origin/destination is AMS and your destination/origin is a Transavia-only airport, then KLM will not sell you a ticket to travel only on these Transavia codeshares. But if you switch from AMS to any other KLM destination, then the KL codeshares on the Transavia route may be offered.

And it is not necessary to book under a KL code to earn miles and XPs. Have a look at the Transavia partner page on the FB website.