So, you're right: you're foiled because the Lumix FZ18 isn't an interchangeable lens camera. The adapter you link for is for Panasonic's Micro-Four thirds cameras, like the Lumix GF1, which do use interchangeable lenses. It kind of confusing for Panasonic to use the same branding for both those cameras and their attached-lens models, but there it is.

The adapter is designed to go where a "native format" lens would, and in this case simply to adjust for the physical connection difference and the different distance in the distance from the back of the lens to the film/sensor plane — it's bigger in SLR designs than for mirrorless cameras, which is why you can get an adapter like this in the first place.

But with your camera, there's already a lens, and while it's possible to get "secondary lenses" (like this) meant to attach in front of another lens, you can't just take another lens and do that. It might be possible to design an adapter with adjustment lenses of its own so that you could attach DSLR lenses, but the results would just be a novelty.

So, there's not much you can do except hold on to those lenses until you do have an interchangeable lens camera. Pentax makes some nice modern ones with which they'll work like a charm, and you wouldn't need an adapter. Or, you could get a Micro-Four Thirds camera from Panasonic or Olympus (or similar-but different NEX and NX cameras from Sony or Samsung) and use an adapter. There are very many great old Pentax lenses out there, so K-mount adapters are common.