You are free to make quests as you see fit but typically writing quests involves creating a dialog and attaching NWN scripts, to each dialog line, to control when they should be shown and what action to perform when they are selected. The only major limitations on Sinfar is that server resets will erase any state information stored on the PC local variables. If that is a problem, you will need to store the information on either inventory items or using Sinfars custom journal system.
There are many external sources for information on writing quests but here are a few. If you are unfamiliar with the NWN quest system, it is recommended that you look at least one of these.
If the above seems like a lot of work, especially if you are unfamiliar with coding or NWN script then there are some Sinfar specific alternatives that remove or reduce the requirement for new NWN scripts.