Extending a JR Kansai Wide Area Pass

I plan to go from Shin-Osaka Station to Kanazawa Station on the last day that my JR Kansai Wide Area Pass is active.

I plan to board a Thunderbird Limited Express train bound towards Kanazawa and take a unreserved seat. The JR Kansai Wide area pass will allow me to get on the train and ride it as far as Tsuruga, but not all the way to Kanazawa.

I am willing to pay to just stay in the same seat, but my question is: how much will it cost me? Here are unreserved seat fees:
Shin-Osaka -- Tsuruga = 3990 yen
Tsuruga -- Kanazawa = 3990 yen
Shin-Osaka -- Kanazawa = 6930 yen

Will I pay a seperate ticket fee of 3990 yen, or will I be able to pay just the extra 2940 yen to extend my trip?

Can this trick of extending the range by paying the cost difference also be used to pay for a shinkansen extension from Shin-Osaka to Hiroshima?

My guess is that the answer is that I will need to have a separate ticket for the additional leg of the journey, but I thought I would ask the question.

If it is possible to pay to extend my journey, should I visit the ticketing counter at Shin-Osaka station before I get on the train, or, should I pay the ticketing conductor on the train?

I am willing to pay to just stay in the same seat

That cannot be guaranteed because you can take only non-reservation space with this Pass.

My guess is that the answer is that I will need to have a separate ticket for the additional leg of the journey.

I presume it�fs a correct answer.

should I visit the ticketing counter at Shin-Osaka station before I get on the train, or, should I pay the ticketing conductor on the train?

I suppose you can do the latter. However, regardless of whether you pay for your pass outside or inside Japan, it seems you will have to visit a JR-West office at least once to get the Pass, which can be your chance to confirm about your questions.
- http://www.jr-odekake.net/en/jwrp/kansai_wide.html

Please note that in Shin-Osaka there are also offices of JR-Central, which is using its orange JR icon and operating Shinkansen between Shin-Osaka and Tokyo.
When you have to wait, be sure that the queue leads to a designated office of JR-West, which is using its blue JR icon.
- http://www.japanrailpass.net/images/logo/jr_05.gif

Each of the fees you have listed consists of a base fare and a Shinkansen charge.

A JR regular non-reservation express ticket is valid for two consecutive calendar days.
If there is a vacant seat to be reserved, you can once (but only once) switch from a non-reservation ride to a ride with reservation by paying for the gap (even on the train), but you cannot vice versa. A gap in the charges varies from 310 yen to 710 yen.

-Will I pay a seperate ticket fee of 3990 yen, or will I be able to pay just the extra 2940 yen to extend my trip?
You need to pay 3990yen.

-Can this trick of extending the range by paying the cost difference also be used to pay for a shinkansen extension from Shin-Osaka to Hiroshima?
Yes, it can. You need to pay the fare for Okayama - Hiroshima.

-My guess is that the answer is that I will need to have a separate ticket for the additional leg of the journey, but I thought I would ask the question.
Your guess it right.

-If it is possible to pay to extend my journey, should I visit the ticketing counter at Shin-Osaka station before I get on the train, or, should I pay the ticketing conductor on the train?
Both ways works and the fare is the same at the ticket counter or on the train.

Thank You, for your replies. I will probably wait until after I pass Tsuruga Station, and just pay the ticketing agent on the train.