In your example "All" would have to be a data point that is associated with all values. This means you have to work the value "All" in to your data model in a way that it presents selecting all values. I dont think this will be a intuitive or logical for the business user, so I would advise on using the native Qlik Sense functions instead.
Educate the users to utilize the powerful selection options in Qlik's associative engine. In the field or dimension context menu, simply select "Select All" to select all values or "Select possible" to select all the current white values.