seat on train

if i purchased the non reserved train,then it is no seat at all,can i buy the seat ticket on site?

If you use a Japan Rail Pass: no.
If you do not use rail pass, then it is usually possible by consulting the conductor.

then if i see there is a vacant seat on seat-reserve train, i can pay the conductor to change or seat to reserved train?

If there are vacant seats you can seat anywhere even without reservation...
What do you mean : "no reservation train-no seats at all"???

Yes

I mean i will buy the non-reserved ticket with hoping to get the free seat.but if i dont get the chance, i willing to buy the seat on the reserved car train.is it true i will only add the seat-free that's same if i buy it from the station?

is it true i will only add the seat-free that's same if i buy it from the station?

Usually yes (assuming you don't switch from ordinary car to green car). Some trains, such as the Nozomi, will require an additional few hundred yen on top of that.