Not entirely sure if I understand your question "
Is this possible to fetch the Product list from our Database without updating the Sitefinity Database...
Yes of course it's possible, you can create a custom control, drop a data source on it and render it out on a grid, but that doesn't make 'your' products 'sitefinity' products. If you want to have products that can work with the shoppingcart and checkout process, you'll need to import them so they'll correctly register as 'sitefinity' products.
With regards to your dll as a payment provider there are 3 routes:
1) Modify your .dll so it'll match the Sitefinity provider system and afterwards sell it on the marketplace.
2) You could break open the current checkout/provider system and inherit from it, insert your new logic to match the .dll and build a custom checkout control to take the new .dll into account (which will cost you tons of hours to test with each upgrade)
3) Settle for the low budget/no time solution and use either the pre-processing hook (introduced in v5)
or the post processing hook. Personally I'd opt in this case for the 'post' by creating an 'offline payment method'. And then on the redirect page (which you can set in the checkout widget) insert your custom code to use the dll.
I'm always in favor of not prying open a working system so I'd choose either option 1 or 3, depending on the budget and how serious you guys are with future Sitefinity projects....