Hi there,
Of course it is possible, but where exactly would you need to place your sign? Most often, you can use the format or formatter property, to create a custom formatting for tooltips, dataLabels etc.
Take a look at the dataLabel example, with a plus sign in front of the value:
https://jsfiddle.net/BlackLabel/b8ey35gk/
Do not hesitate to contact us,
Best regards!