Amarjit, an alternative way would be to hide the textbox. The 'Visible' property is bindable, so when the user clicks the 'update' button like you mentioned, that could trigger the visible property and make the textbox visible. If you want to use ReadOnly, that would indeed be a custom feature.
There's a workaround with 2 textboxes, one readonly and one editable which are on the same place in the Composer. Click on Update button will hide the readonly textbox and show the editable textbox while enabling and disabling Visible properties.