What would happen if a planet's gravity suddenly increased

This may sound like a bit of a stupid question, but what if a planet became heavier? By suddenly, I don't mean in a split second.
But, say, if someone wanted to (for someone writing sci - fi) change Mars' gravity closer to that of Earth, could they theoretically bring some heavy material, like white dwarf star material, onto the planet then drill and place it by the planet's center? Would this destroy Mars' orbit, what would this do it's moons, etc.
Thanks for answering this stupid question o' mine.

Theoretically it would be possible for someone to drastically alter a planets gravity, although I'm sure it wouldn't be very easy. If Mars became, say, twice as massive in a relatively short period of time the results would be catastrophic. Mars would decrease in size as the matter that it is made up of is condensed by the increased gravity, and its moons orbits would be severely disrupted. It would really depend on how much more massive Mars became as to whether or not its moons would become so much closer as to impact Mars.I doubt that Mars' orbit around the Sun would be directly affected by any increase in mass, unless it increased so drastically as to pull the Sun closer. Unfortunately for you, I lack the imaginative capacity to think of a way for someone to actually go about increasing Mars mass in a relatively simple way.

Hours and days would be shorter, because we'd be going faster around the sun. And! lots of ppl would hate what they see when they step on a scale even more. That'd be a shame.