Thank you for your answer
How can we do it manually ?
Open dt30, then open one of the stadium files stxxx.cpk you want to modify, look for folder "effect" inside stxxx.cpk. Inside effect folder there's effect_config files, eg effect_st016_df.fpk, effect_st016_dr.fpk etc for day fine, day snow, etc, different time and weathers. Extract some of those fpk and inside there are effect_config.xml files which include data for that stadium and weather, and which we edit to have the flares or other things in stadium during game(you can also disable rain and snow this way, or enable).
When you're done editing, put xml back into fpk and make folder path Asset\model\bg\stxxx\effect\#Win and put the edited fpk inside and build a cpk. Of course it would be a bit faster if stadium server existed for PES 18/19 like it exists for PES 17 because this also works for PES 14-17, but it doesn't, so you'll have to bother with cpk's. And as it's also based on stadium and weather/time conditions, it'll look the same on day fine san siro whoever plays there. Unless like i said we have some "server" in the future to relink per club etc.
The above part with files may sound complicated but it isn't, much longer is the part where you manually have to add each flare, smoke bomb, strobo, confetti or text.

There is probably a way to add scarfs, waving flags and choreography for full atmosphere too(they are in south american leagues and CL/EL cutscenes), but they are inside dt12 cutscene files which are .gani and i don't know how to open them. (Unless they are somewhere else too).
In short, you have to find the exact x,y,z coordinates in stadium where you want to place each flare, and you can also pick it's color and size, it's life, it's speed, etc(strobo is fast, smoke bomb is slow for an example). Theoretically you can make one large smoke bomb to cover the entire stadium side, or you can add 200 of them of smaller size and they do the same trick of covering the entire stadium(if that's what you want to do, options are unlimited). You can even place flares on the pitch, but it'll hurt your playing visibility.
Anyway, i may write a little guide on stadium atmosphere when i get PES 19 around december if anyone is interested, someone might find it useful for creating a patch, or just for clubs they are playing with in exhibition or cup or master league.