Going beyond rail pass boundaries?

I am thinking of buying a Kansai wide rail pass but was wondering if it's easy enough to pay extra to go further than the pass boundaries without having to get off the train? For example I want to go from Kanazawa to Osaka but the Kansai pass only kicks in at Tsuruga. Also I want to get from Osaka to Onomichi but the pass stops just beyond Okayama. Can I just buy the extra portion of the ticket at the station but then take the same train as I use the pass on? I know there is a wider Kansai pass that takes in Kanazawa but that is over 7 days and doesn't work out to be much cheaper than paying one-way tickets.

Yes you can. You just show your pass at the time of purchasing your tickets, and the attendant will figure out the amount you need to pay.

You can pay the extra where you disembark. Show the pass to the attendant at the turnstile gate, and he will figure out the amount you owe.

It's really better to figure out the uncovered portion and purchase the corresponding ticket by yourself in advance; it saves everyone's time.

I want to go from Kanazawa to Osaka but the Kansai pass only kicks in at Tsuruga.

In this case, will you already have the Kansai Wide Pass when you get on board at Kanazawa? If you don't have the actual pass, it will be problematic. Correct me if I am wrong, but as far as I know you must have the actual pass, not the MCO, to use the pass. You should check where you can get the pass.

https://www.westjr.co.jp/global/en/ticket/pass/shop/pdf/JRW_EN.PDF

If you can not get the pass before Kanazawa, the only option is to get off at Tsuruga and purchase the pass there, and take the next train.

Thanks for the responses, I was intending to buy the pass before I leave Australia and then get it validated once I've arrived in Japan but I thought I could do that in Tokyo, I didn't realise I'd have to be in the area covered by the pass to do that. Maybe I can get it validated in Kanazawa seeing the JR Kansai Hokuriku Area Pass also covers the Kansai area? I will check that, thanks for the tip.

No, you cannot exchange a Kansai Wide Pass MCO in Kanazawa. And you cannot do it in Tsuruga either; in Tsuruga you can only buy a new pass.

The JR West website clearly states which station locations you can buy and/or exchange passes and/or exchange orders. It will only take you 15-20 seconds to find your answer.

https://www.westjr.co.jp/global/en/ticket/pass/

Thanks everyone, that table answers my question.