Firstly, thanks for a quick reply.
I did pretty much what you described - i converted the particles, removed the emitter, selected grass blades, selected active selection, pressed control-j and ended up with much more grass blades than before - and this is exacly my problem, I do not want to have more blades than are in the file right now.
The file I uploaded has only grass, since I didn’t want to make people download a large file with stuff that isn’t relevant to the question.
I do not need to modify those blades any more, I need to be able to export the model into Unity as one mesh - there’s arount 1600 objects here and it damages the performance. I know that in a normal case subsurf on grass is overkill, probably grass that is more than a plane would be too much too - but here the case is that the camera is very close to the ground, so the grass takes up about half the height of the screen.
So, if i did not make myself clear before - I need to have the models that are in the file joined into one mesh, and pressing ctrl-j makes more grass blades appear.
By the way, did joining the objects work for you fine? If that’s the case, the problem may be my Blender settings, I thought that the models were linked in some way I did not know of.