Probably it will not work. The problem is that gas molecules have a wide distribution of velocities; most molecules in a gas are not really close to the rms velocity. Over the course of billions of years we should expect faster molecules to escape. At some point all the molecules have become "faster" at one time or another and make a run for it.
To prevent that, the body's gravity must be strong enough or its temperature low enough (e.g. Titan) to make the gas molecule R.M.S. velocity well below the escape velocity. This chart shows which gases are retained, primarily on bodies much larger than Ceres. Xenon, for instance, could be retained on the Moon if it were available, and $\text{SF}_6$ with a similar molecular weight would behave similarly. But on Ceres the escape velocity is below the range if the chart and that, combined with typical inner Solar System temperatures, would put the data for Ceres far below the boundary even for the heaviest gases. We therefore have no chance of retaining a $\text{SF}_6$ (or any other plausible gas) atmosphere on Ceres by gravity.
Moreover, $\text{SF}_6$ is a powerful greenhouse gas, so even if it could be retained we would have to limit it to trace amounts or risk adverse effects on the ice-covered dwarf planet/asteroid.
Appendix: How much of this stuff is there, anyway?
Another issue is that there just isn't a lot of sulfur hexafluoride around. The US Geological Survey recently reported that sulfur hexafluoride gas increased markedly in Earth's atmosphere ... to over 4 parts per trillion; and that is "primarily of anthropogenic origin" (quoted from the abstract of the reference). Earth atmospheric xenon is much more abundant than that according to Wikipedia, reaching parts per billion levels. Sulfur dioxide, another relatively heavy gas we Also see in the atmospheres of Venus and Io, is also more abundant on Earth. A corollary to these findings is that putting even a few Pascals of sulfur hexafluoride pressure around Ceres would require far more sulfur hexafluoride gas than is available on Earth.
The above data are for Earth, but they are likely to apply at least roughly to other bodies in the inner solar system where hydrogen and its compounds (aside from the Sun) are relatively poor in abundance. Comparing sulfur hexafluoride with xenon this would be all the more so if the sulfur hexafluoride on Earth is indeed primarily man-made.
