how can we modify sourcing & filtering for a field that has been created by UI through SS2?

i think its not possible to change through script

A way around this is: 1. On Edit / Create Beforeload add a Custom field to the form and hide the field you added via the UI. 2. Filter the custom field as appropriate (you can use client script to populate this this). 3. On Before submit, capture the value of the custom field and use that to set the UI added field. Leave your UI added field unfiltered so there's no collision with you want to set it to on before submit.