Yes, the general steps would be the following:
1. Create your secondary data source to retrieve data from your SharePoint list. (no qRules) You will need to make sure you bring in the ID field.
2. Look at the data source brought in by InfoPath. Now create your schema in the main data source, so that the fields match the secondary data source.
3. Copy the data from the secondary to the main data source by using the CopyTable command.
4. Create the SharePoint list mapping. Hilary has a good series of blog posts on this, linked from here. Make sure to use the /id parameter so that the existing items are overwritten. The new items won't have an ID, so those would get added to the SharePoint list by the qRules command.
We also have free qRules training available here.
The full list of qRules documentation can be found here.
Hope these help!