Do you want to add your own reason codes to the Finesse configuration? If so, they you use the WebServices API for that. There is no gadget javascript for updating configurations which require admin credentials to update.
Do you want to have your own reason codes outside of Finesse? You cannot do this for NotReady reason codes.
in order for Not Ready reason code to be assigned to a User Not Ready state change, it must be configured in the Finesse system.
You can put anything you want for wrapup,(wrapup can be updated like any callvariable in UCCE) and you can retrieve your own wrapup codes from your own server.
If you are maintaining your own list of wrapup codes somewhere, say on your own server, if that server is accessible from the Finesse server you can use
Gadgets.io.makerequest (see the Finesse Developer Guide section on APIs available to gadget JavaScript
https://developer.cisco.com/docs/finesse/#!apis-available-to-gadget-javascript
There is an example of using gadgets.io.makerequest in the WebServiceSampleGadget available on DevNet Finesse tech center in the 10.0.1 sample gadgets folder https://github.com/CiscoDevNet/finesse-sample-code/tree/master/WebServiceSampleGadget
Here is a link to the sample gadget https://developer.cisco.com/docs/finesse/#!sample-gadgets
