I'm not overly familiar with text in Painter, as I normally do all that in PS or illustrator, but if you don't need the text to remain editable then there is a workaround.
Convert the text to Shapes from the Layer drop down menu. Open the Shapes Attributes dialog and select stroke if it's not selected already. You can alter the width and colour.
Alternatively, you can select the text and convert the selection to shapes, etc. that way you still have editable text. You'd just have to do the process again if you did edit the original text.