It is possible to create an improved plugin from 2 other plu

An example:

Using a copy of the original IAP plugin and the plugin CocoonJSads is possible to make a new plugin "IAP2" adding the "Purchase product with preview" of CocoonJSads that is more elegant because displays the information before purchase.

Another example:

Place all Plugin GooglePlay commands in the plugin PhonegapGame 1.0.63

because in PhonegapGame lack a lot, for example I can not verify that an achievement is unlocked.

Needs a lot of knowledge in JavaScript?

Without digging into this, my hunch is that you wouldn't be able to do what you are saying. I don't know much about either of those plugins, but they each serve a specific purpose. I think it would be like trying to control physics behavior from the platformer behavior. If you wrote something from the ground up, it could be done if the platforms support it, but honestly, you probably need to open the plugins in a jscript editor and have a look. 9/10ths of the work would be to understand what is being done and how, and then to see if it could be supported as one plugin.

-1 to me for a very vague answer... lol

TLDR: you need to know jscript in and out to do this and know the platforms.