Where I can change drawen curve for a curve with calligraphic stroke. No draw new curve with calligraphic, but how can I add to existing curve calligraphic stroke.
Excuse my simple questions, I use Inkscape only 3-4 days.

To add to an object using you can select the object and hold shift as you draw. Sorry if that's not what you're asking.

I decide to add calligraphic stroke later , when I have drow curve yet. Is it possible to change stroke to calligraphic stroke post facto?

The nearest you can do is add a path along path Effect to it to stretch a curve down it.
Its easier to do in the development as you can apply it as an LPE.

Thanks, Simarilius