I noticed this post was done on the 1st but just in case you still need help ...
I did something similar to what you're doing and what I did was set up a variable to save the current focus then use the onSetFocus function on each of your text fields to set a new value for the variable e.g.
//Set initial variable value (initial focus)
var FocusedField = textField02_txt;
//When focus is on field set new variable value
textField01_txt.onSetFocus = function () {
FocusedField = textField01_txt;
};
Then on pressing the button to insert a character do something like this:
button_btn.onRelease = function () {
//Send the focus back to the last focused text field
Selection.setFocus(FocusedField);
//Show the caret at the end of the text
Selection.setSelection(FocusedField.length,FocusedField.length);
//Add character to end of text
FocusedField.text = FocusedField.text + "a";
//Show the caret at the end of the text
Selection.setSelection(FocusedField.length,FocusedField.length);
};
Hope this helps.