Hi,
this is a tricky question.
If you are qualified, got the 'certificate' that you are qualified for doing a specific job,
you can ask the employer for a
general pay rise. (as you are more qualified and useful to the practice). This will then cover the smoking cessations or other, that you are doing.
If you don't get a pay rise, ask the dentist if you can be paid a bit more for the extra you are doing, but it is part of a dental nurses job anyway.
I am not so sure that there is a general rule for this, possibly only advice.
The dentist is sort of paid, indirectly for smoking advice, as a check up includes checking for gum problems and caries, to take x rays if needed, scaling et cetera, et cetera, as well as smoking advice. They are not paid extra for it.
So, it will be down to your employer if you should be paid more for doing this.