Are all accessories for the DSC W200 compatible with the new DSC W300?

adaptor
convertion lens
slave flash

Stands to reason that they would be. But then again, I'm against adapters and teleconverters from the very start. The problem is that A 2x teleconverter will magnify the images coming into your lens by 2. The downside is, that this will cause your camera to have to be compensated in it's exposure settings because of the loss of light by 1-2 stops. And depending on the quality of the glass in your teleconverter, you may also run into clarity issues. You will certainly run into some "barrelling" issues at the extreme end of your focal length. this will make the image look like you're looking out of a barrell. This can also happen with wide angle converters ass well.

To be honest, while you can purchase these options to add onto your existing lens, I don't recommend them because the tradeoffs involved simply outweigh the benefits in my mind.