Regarding the first question, you can use this code to add new records to the existing Call logs content provider:
public static void insertPlaceholderCall(ContentResolver contentResolver, String number){
ContentValues values = new ContentValues();
values.put(CallLog.Calls.NUMBER, number);
values.put(CallLog.Calls.DATE, System.currentTimeMillis());
values.put(CallLog.Calls.DURATION, 0);
values.put(CallLog.Calls.TYPE, CallLog.Calls.OUTGOING_TYPE);
values.put(CallLog.Calls.NEW, 1);
values.put(CallLog.Calls.CACHED_NAME, "");
values.put(CallLog.Calls.CACHED_NUMBER_TYPE, 0);
values.put(CallLog.Calls.CACHED_NUMBER_LABEL, "");
Log.d(TAG, "Inserting call log placeholder for " + number);
contentResolver.insert(CallLog.Calls.CONTENT_URI, values);
}
(Code taken from Google Voice Callback for Android)
Remember to add the permissions in the Manifest
<uses-permission
android:name="android.permission.READ_CONTACTS"></uses-permission>
<uses-permission
android:name="android.permission.WRITE_CONTACTS"></uses-permission>
Regarding the customization of the call logs database, I do not think is possible.