apps data to sdcard for non-rooted phones (NOT installing apk to sdcard)

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Hi everybody.

As I'm already pointing out on the title of the topic, I want to assure you: no, I'm not asking how to install applications to sdcard.

I'm just thinking about some applications that are already there in the market (e.g. Asphalt 5, iCommando, Space Physics etc.).
Those applications are splitted into 2 parts: the first one is actually the *.apk, the core application, which wants to install into the internal memory of the device. The second part is a bulky datapack which is installed apart, on the sdcard, as an "expansion" of the core application.

Now, I wonder: is it possible to apply this concept to other apps?

Sadly, it's usual that the more an app gets cool and rich with fancy graphics, the bigger the apk results. But after all, bitmaps and audio files could be easily stored into the sdcard instead of the limited system's memory...

This would be convenient for at least two reasons:
1) this would work for everyone, even non-rooted devices;
2) you don't need to repartition your sdcard to do this.

I'm now already playing with a bunch of apks, trying to mod them in order to reduce the executable core to the minimum and to make them point to a defined directory in the sdcard, containing data files.
No success for me, till now

Had anybody ever tried to do what I'm trying?
Any advice would be greatly appreciated, thanks

Not anyone? :/