Editing the built-in autocorrect dictionary in AOSP keyboard
This other SE answer might be of help if your phone is running under CyanogenMod. That answer, in turn, points to this article where author shows the location of built-in dictionaries on the phone filesystem, as well as the location of built-in dictionary source files in Android open source project.

The rationale (all to be tried) should be grab the source of a builtin dictionary you want to edit, modify that, generate a new APK file out of that, and finally substitute the original dictionary APK with the custom APK.