Is it possible to put Micro-transactions in game?
Is it possible to create DLCs and add-ons and sell them separately?
An ingame online connection (either for microtransactions or for DLC) is very difficult.
Yes, it's doable (and probably easier to do than online-playing which is also often asked for), but it will require a lot of work, especially because you'll need a secure internet connections programmed inside the game engine.
In my opinion that is not something that can be done as a hobby or for free - and the professional programmers on this site who could do this, will probably ask for payment in the area of several hundred dollars at minimum.
Add-ons are possible and have been done before - there is even a patch-script on the master list. However, that requires you to plan ahead and prepare the game for those add-ons before releasing it. Because there are several aspects in the game that can't be changed easily and you would invalidate all older save files with certain forms of updates that weren't planned from the beginning.