I think you have the right idea just the wrong way of getting it done.
yeasts eat sugar and excrete alcohol and CO2. When naturally carbonating (using the CO2 from the yeast) the vessel needs to be air tight and able to withstand quite a bit of pressure. I will give a basic rundown as follows.
1- Make the wort (we will assume you did this right)
2- Pitch (add yeast)
3- Primary fermentation (there is no extra sugar added normally)
4- Optional secondary fermentation (This is for clarification mostly and you do not add sugar that I know of because that is kind of defeating the purpose of clarification, yeast eats sugar and multiplies.)
5- bottling, mini-kegging or carbonating and storing by some means. Usually there is a small amount of priming sugar added at this point and then the beer is kept in a slightly warmer place (70+ degrees) for a few weeks and here is why. There will be some yeast still living in your beer, by adding the sugar, bottling and then capping/sealing the vessel, the CO2 has no where to escape to and pressure builds. As this happens CO2 is forced into the liquid making bubbles or carbonation. This is normally called "bottle conditioning". Next there is the "cold crash" this is where you take the conditioned bottle and place it in the fridge or cold place that is above freezing and the cold temps actually help the CO2 get into the liquid adding more "carbonation" and making your beer cold!
Normally the primary is 2 weeks, the secondary is 7-14 days and then bottle conditioning another 2 weeks is the minnimum.
Many people here do not use a secondary fermentation and just do a 3 week primary followed by a 3 week bottle conditioning.
The correct way to determine when primary is done is actually NOT by time but with a hydrometer, getting the same readings for 2 days in a row.
The reason that your post concerns me so, is because you seem to be slightly confused on the secondary and bottling stages. If you add more sugar to the beer and cap it in a demijohn it is possible that the demijohn will NOT be able to withstand the pressure of the CO2 and they may crack, leak or even explode (Think balloon popping not ball of fire)!! resulting in lost/spoiled drink and a giant mess.
IF the demijohn does not turn into a "bottle bomb" then when you open it you will need to drink all the contents quickly as the beer will go flat (loose carbonation) quickly.
I hope this helps!