If you're concerned about what exact variety of vegetable you get from the seeds you planted, then you may be disappointed.
It's a little like air layering trees. If the parent tree is grafted into dwarf rootstock, the tree you get from air layering above the graft will be exactly what the scion is, but will NOT be a dwarf.
If you're not concerned about it, go for it, and let us know. The Growing Other Stuff thread I started is great for these things. I'd love to see what turns up.
I'll use tomatoes as an example to start with.
A variety of yellow tomato called Lemon Boy is exceptionally good in my climate. Heat tolerant, drought resistant, and prolific fruit production all season long. Can be tricky to find, though because it's an unstable hybrid of two other varieties. That means that if I plant the seeds, I'll get something other than Lemon Boys.
However, I will still get tomatoes, and they will in all likelihood be edible. Palatable is another question all together, but they'll be nutritious in some way.
Peppers- capsicum anuum specifically, the species most commonly grown for Food production, and includes pretty much EVERY variety of pepper you'll find in a grocery store- are notorious for cross pollinating very easy because all the hundreds of varieties are all one in the same species.
So if you plant your sweet bell pepper next to cayenne pepper, not only are your bells likely to come out spicy that year, any seeds you plant will produce a hybrid of the two peppers.
A related side note: ANYTHING you plant too close to peppers runs the risk of producing something with a bit of the pepper characteristics. Once as kid my mom planted the cauliflower next to the jalapenos and we got spicy cauliflower. It was delicious
We put it in our pickled pepper mix with an egg in each jar, thus kicking off my love of pickled eggs.
If you want to ensure you'll grow the same thing you got the seeds from every year, look for what's called heirloom varieties. These are individual cultivars, not hybrids, so the seed will make the same thing every time you plant.
Back to tomatoes, the San Marisano variety is an heirloom tomato. It's been specifically bred to produce fruit with specific qualities, and only pollinates with tomatoes that will result in San Marisano fruit when the seeds are planted next year.
I can't speak to your hokaido pumpkins specifically, but butternut squash is an heirloom, so you'll only get butternut. I don't bother buying seeds for those, just a squash at the store.