For extrasolar planets very close to their host stars - about one-tenth Mercury’s distance from the Sun - this gravitational pull eventually tidally locks the rotation, and the length of the planet’s day ends up equal to the length of its year. (The Sun’s tidal distortions make tides in Earth’s oceans.) The mass in that tidal bulge feels the gravitational pull of the host star, which tries to keep the bulge pointed toward the star. Which means big, expensive thrusters and the risk of ripping the entire planet apart and turn it into a molten ball of volcanic madness.If a planet orbits extremely close to a star (as many extrasolar planets do), the star’s gravity stretches the planet into a shape called a prolate spheroid with a tidal bulge that somewhat resembles a football. The problem is, you have to overcome the attraction between the tide bulge and the star, so this will require some serious impulse from your planetary thrusters. You can also replace the atmosphere afterwards (or store it for the duration).
Keeping the contraption stable and in one piece will require some work, but I assume that's the kind of small engineering problems that won't stop you. You may want to build a geostationary ring around the planet and link it with the surface with space elevator cables and put thrusters on the circle, in order to avoid blowing the atmosphere away. The obvious solution would be to put a giant Catherine wheel of angled thrusters around the Equator, and start making the thing rotate. The question doesn't seem to forbid technological methods, so there are a few options there. The high speed and large mass of the rogue planet also play havoc with the local gravity and after a (geologically) brief period of orbital wobble, the Super-Earth settles down to a more normal rotational cycle. It's going way too fast to end up in orbit, or even be in any danger of actually colliding, but the asteroids it gathered up start bombarding the planet. Then it swings by your planet at high speed, grazing the atmosphere. One the way in, it shoots through an asteroid belt or three and sends rocks flying in all directions, even pulling a few along in a game of follow-the-leader. Perhaps a rogue planet has just shot through the system swerving dangerously close to your Super-Earth. If your bombardment takes place over a long protracted period, with many many rock causing glancing blows, perhaps even aerobraking in the atmosphere before impact, then over many hundreds or thousands of years, they may impart enough momentum into the planet to start a chain-reaction wobble in the orbit, which in-turn may cause the planet to rotate on it's own. Why can't this be the cause of the rotation? If we remember that a tidally-locked planet is already rotating, only at the same speed as it traverses around the star, then we only need a relatively small amount of acceleration to unlock the planet. You mention in your question that the planet suffers a bombardment. If anyone has numbers for any of the stuff I completely made up, please comment and I'll change them to actually correct values. I'm not sure what that level of radio waves will do to a planet, and I doubt it would be pretty or at all nice to any life present, but it seems to work to restart the spin. So, in conclusion, my worked example with half the numbers made up and most of the other half being Fermi estimates seems to work. We can assume an energy transfer of, let's randomly say 0.00001% because of most of the pulsar beam missing the planet and some of the energy being transferred to heat and translation instead of rotation, that moves the required time up to about 68 million earth years, still within your hundred million year timeframe.
In my answer to another question, I suggested that the Super-Earth in question be tidally locked to its host star for a period of time while part of its surface experienced a bombardment.
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