Do you mean to use a Salesforce report as the underlying data source in lieu of a database table or SharePoint list? From my initial test it does not seem possible to use a Salesforce Report as the datasource. You should check with Nintex support about this to be certain.
However, you could use the Salesforce "Query Record" and "Retrieve Record" actions to accomplish a mail merge using Salesforce data. Use the Query Record action to fetch which records you wish to work with. (You will need to specify the API names for the objects (e.g. Contact or Account) and for the field names that you wish to filter on.) The Query Record action will return a collection of Record IDs.
You can then iterate on each Record ID in Collection variable using a For Each action. Within the For Each action, do the following:
Which SharePoint platform are you using (Office 365 or SharePoint 2013/2016)? Note that the specific configuration of the Salesforce actions and the Document Generation action is different for Nintex for O365 compared to the on-prem editions. If you are using N4O365, you'll need the Saleforce Security Token, which is not needed for On-prem.
Hope this helps.