fz18 can take 46mm(extending barrel) and 55mm(outer fixed barrel)
in order to use a macro lense, or teleconverter, you'll need an adaptor tube. panasonic has this as an optional part(DMW-LA3) but it's very hard to find in shops. you can also get third party brandless tube outside at cheaper price.
now comes the problem of using this type of tubes. they cause vignetting at wide angle aka 28mm. and in order to shoot without vignetting, you got to remove the tube. then one smart alex from dpreview.com came out with an idea of this peramaal adapter tube, which if unused, can be "shortened" by turning the tube inwards, thus solved the vignetting problem.
there are quite a few guys who bought this peramaal adapter and had them permanantly fixed onto their fz18. i'm one of them.
by using this peramaal adapter, it'll also step up the thread size to 62mm, as it's code name PA62E suggested.