This wouldn't work, as the size and power of an electromagnet powerful emough to pull plasma off of a star, and then reaccelerate it to use as a plasma thruster would be larger than earth itself. It's far easier to just use a VASIMR or solar sail. And then there's just the small issue of the plasma melting your spacecraft, but that's a really small problem...
SomeWhat a good Idea, but the hydrogen would already be on fire.