Can I somehow convert .pkg files to .apk format.
It is difficult to provide correct details referring to a .pkg to .apk conversion. Numerous programs/utilities seem to be using the .pkg suffix. It does, however, most likely relate to an installation package file. These .pkg file types don't seem to be directly convertible to the Android Package File format, .apk.
If you have a software installation package for one operating system/platform, find out whether the developer also provides an Android version. That could be your only option.
The database currently does not contain any pkg software converter.