Hi there. Nope, not currently possible (and I won’t be adding the feature because the plugin is basically in “maintain until this version of Widgets goes away completely” status).
(An awkward workaround would be creating Categories just for the specific post and only assigning it to that one Post.)
You could check if Widget Logic (https://wordpress.org/plugins/widget-logic/) still works. That was the plugin I used to recommend when people wanted functionality outside the scope of Conditional Widgets.
Good luck!
