Short answer: you can't. But I think you have a couple options.
Try using a custom Content Type rather than creating columns directly on the list. When using a CT, you can make the backing fields (i.e. the 'decision point' columns) hidden so they don't appear in forms. This could still produce more 'clutter' than what you're comfortable with, though.
You can also try creating a secondary data source based on a simple XML document. I think this is the method you're looking for. I've only done it one time, but it worked well in my not-too-complicated use case.
First create an XML file, something like this:
<?xml version="1.0" encoding="UTF-8"?>
Here I've given myself three fields - valueA, valueB, valueC - for storing my 'decision points' inside a single group. Multiple groups may be allowed but I've not tried that.
Next, create a data connection to Receive data from an XML document. You are required to include the file as a resource (which you want to do anyway) when working with SharePoint list forms. And since the XML is basically empty, you might as well load the data on form open. That's about it for the data connection. You can set initial values for these fields in a Form Load rule, if necessary.