It's quite simple to do what you've described. Simply enter the following code as the field's custom Format script:
if (event.value>0) event.value = "+"+event.value;
This will cause the field to show a plus symbol before positive numbers, but will not change the actual value of the field so you could still use it in calculations. The minus-symbol is of course an integral part of a negative number, so there's no need to add it.
I would use the "util.printf" method in the custom format script. You need to set the cFlags parameter to "+".
For example a custom format to show a value with the sign and rounded to 2 decimal places:
var cFlags = "+"; // flag to force sign;
var nWidth = 4; // minimum number of characters to display in result;
var nDec = 2; // number of decimal places or precision;
event.value = til.printf("%,0" + cFlags + nDigits + "." + nDec + "f", event.value); // format the event value for display;