Reading about Stellar Evolution, I see that one type of stellar remnant is a White Dwarf. This is a mass that just sits there radiating heat until it becomes a black dwarf. If enough mass is added, it can become a Neutron Star. Even more mass can tip it past the Chandrasekhar limit to collapse into a Black Hole. But can a White Dwarf ever become a new fusion-powered main sequence star if enough Hydrogen is added quickly, say, through a trip through a dense nebula of H2? Or maybe a close encounter with another star that sucks away a large portion of the H2 from that star?
R S Ophiuchi is a recurrent nova, exploding every 20 years or so as hydrogen is transferred from the nearby red giant companion; see this image by the marvellous David Hardy.
eventually this star might explode as a type 1a supernova.
You could try to engineer this sort of event so it happens in a more controlled fashion - perhaps by diverting a gas giant then ripping it apart so that the hydrogen falls onto the star. But to be honest this is a very inefficient process, and you would be better of trying to use the gas giant in other ways.
Note that one way to use a white dwarf is to use its own luminosity directly. These objects start off very hot, radiating in the high ultraviolet - but over time they cool, until they reach Sun-like temperatures. There are several nearby white dwarfs that are just as yellow as the Sun, and you could build a habitat at a distance of a million kilometers or so away from the star, and take advantage of the yellow sunlight. From such a habitat the white dwarf would look identical to our own Sun, in size, temperature and colour.
If it were possible to find a planet in such a position, it could conceivably be made habitable for a billion years or so - but (unlike our Sun) the white dwarf would gradually get cooler and redder, leaving the planet locked in a permanent ice age.