Q:

Installing third party APK from another App

Currently I'm developing an app and I included an apk that created by third party to be installed on specific condition. This apk is stored on assets folder. The problem is when I tried to install the third party apk, it ask me to activate allow the unknown source setting.
My questions are,

Can an .apk file contain another third party .apk file in its assets directory while uploading to Google Play?
Is it possible to sign a third party apk (make it known source)? So it won't prompt the user to allow the setting.

A:

Can .apk file contain another third party .apk file in its assets directory while uploading to Google Play?   

Yes, I am sure that we can. 

Is it possible to sign that third party .apk (make it known source)? So it won't prompt the user to allow the setting

It is not something about the app signature, but the source of app installation. The only trusted source is the Google Play. All other sources (including assets directory of other APK regardless of where it has been installed from) are not trusted sources. If you want to avoid that problem - prompt to install the 3rd-party app from Google Play.

