if someprop and someotherprop are variables, Richard D's answer is correct, if they are hardcoded strings, you could write your code like this:
var data = {someprop:{someotherprop: 'value'}};
if, however, they are values stored in variables, and you really - really want to cram everything into a single line, try something like this:
var data = {window.someprop : {window.someotherprop: 'value'}};
where window can be replaced by any object, if the someprop value is itself a property of an object:
var data = {someForm.name : { formElement.name : formElement.value}};
Or jQuery objects, since your tag suggest you're using jQuery:
var data= {$('#formId').attr('name'):{$('#formSelect').attr('name'): $('#formSelect').val()}};
Note: the jQuery example is not to be recommended, as the id selector is used twice on the same element. jQuery will scan the DOM-tree twice, so you're better of storing your that #formSelect element in a variable somewhere.
somepropon one line then settingsomeotherpropin that?somepropfirst