I bought a Juniper. During winter, can I make it undergo dormancy by putting it in a dark but warm (around 20 - 25 degree Celsius) location? Thanks.
No; the conditions you describe are highly likely to kill a Juniper - they expect to be cold for some of the year, and a protracted absence of light for a green plant spells death. I'm not sure what you mean by 'dormant', but dormancy occurs during winter when it's cold IF the plant is outdoors and thus exposed to winter conditions. Dormancy in that case refers to the plant not growing during the cold, dark days of winter.
No, the basic cause which triggers dormancy is because the speed of chemical reactions is strongly dependent on temperature.
If a plant is kept warm and in permanent darkness, it will attempt to keep growing until the lack of photosynthesis causes its "metabolism" to get out of balance, at which point it will start to die.