From a (real world) physics point of view, it makes absolutely no sense at all to absorb a star into a planet.
Compared to a star a (rocky) planet has an insignificant mass and volume. Absorbing a star (in the form of its matter) is therefore rather improbable. And even if they found a way to absorb its mass into SKB, they would have the slight problem of gravity.
A star (to be called as such as in having nuclear fusion going on) has a mass of roughly 75 times the mass of Jupiter. If you compress that to a size roughly that of earth, the gravity on the surface would roughly be 10-100 times that of normal earth gravity.
Everyone on SKB would have been pressed flat to the ground and couldn't have moved much anymore.
So absorbing the matter of a star is highly unlikely. On the other hand, energy itself has no mass. So what they could have absorbed actually was the stars energy and stored that in SKB.
If it were possible to absorb a stars energy the right way, it would even fit to what was depicted in the movie. A star is gas in a hydrostatic equilibrium, meaning it is compressed by its gravitational pressure and the fusion power released counteracts that contraction. If you'd absorb both energies (fusion energy and gravitational energy) you'd for once have a HUGE energy source and the star's matter would slowly dim and become a rather compact, heavy and small dark object, that couldn't be seen any more other than with specialized equipment.
The energy stored inside the planet (in whatever the form) would mean a catastrophic potential of destroying said planet in a supernova like explosion.
When so much energy suddenly gets released, it would blow apart most of the planetary body and - if a planetary sized husk remains - that husk would be insanely hot. So hot it would glow bright like a star for some time while continously dimming again due to the energy radiated to space.