"Add choice" functionality

Here's an existing idea that might fit the bill: https://nation.marketo.com/t5/ideas/allow-multiple-levels-and-or-nested-conditional-logic-choices/idi-p/256917

If x and y=true are defining your audience then you may use it directly under the campaign smartlist and send the alert using the flow step. Use AND in advanced filter settings.

In case you want to send Alert to the stakeholders to notify some activity already being done and x and y=true is a choice in that flow step, you may use an external smartlist then.

I'm not sure of your exact use case, but what you're discussing may be able to be done using the filter "Member of Smart List Is: [Smart List]". That way, you set up your Smart List to have the criteria x and y = true for evaluation.