first, selectbox is a listbox? There isn't a "selectbox" input type I am aware of.
If I am reading this correctly, then you would need to make the listbox tv values into an @EVAL binding, and point it to a custom snippet:
@EVAL return $modx->runSnippet('myMIGXValues')
The snippet would need to go get the MIGX tv value for the resource's parent, and parse the JSON value to return it in the proper format : Title == value || Title2 == value2
More on @EVAL bindings here:
http://rtfm.modx.com/revolution/2.x/making-sites-with-modx/customizing-content/template-variables/bindings/eval-binding
As for the snippet itself, getting the TV value for a resource's parent can be a bit more involved. BobRay has a good write up on how to do it here:
http://bobsguides.com/revolution-objects.html He addresses getting raw and processed TV values from different resources about halfway down the page.