Scarow, as Simon explained you need to add objects to maps manually - there is no automatic programme or script that can do it for you. However adding an object that you have saved as a .png elsewhere on your computer is a simple matter of dragging it over the gimp window and it will drop right in. It only takes about a second. I'm not sure why you would want to speed this bit up. After that, you would need to rotate it, resize it, (in my case, recolour it), shadow it, etc: these bits take a bit more time and need to be done by hand.

Also, as you can see on this attached map, i actually create several "maps" for the different parts (on different layers), but the transitions are not very smooth, so if someone could give me an advice how to make that nicer, that would be nice.
Where the transitions overlap, it is quite easy, and you have two options:
(i) simple: use the eraser, select fuzziest brush, and carefully delete parts of the upper layer where it overlaps with the lower layer to create a subtle transition.
(ii) advanced: use layer masks on one or both layer to create the same effect as (i) but you can get it to look better.

Where the transitions do not overlap, you will have to draw a new section yourself to create an even transition. You can use the clone tool to make this a lot easier.