I think adding pineapple juice to the sauce will make for more problems, - it may just separate on you,
but you could try it and find out.
As for sweetness, just add a bit more sugar to your sauce.
I've made plenty of pineapple and ham pizzas, and honestly, I've used just my plain old pizza sauce, and it's tasted
very very good.
I drain off all the pineapple juice from the can, and then sprinkle on the chunks moments before the pizza goes into the
oven. I've found that if I don't drain the pineapple very well, it make the pizza very runny, and my kids didn't like it,
nor did I.
Any way you make this pizza, it's going to be "hamn" good !