I'm not familiar with Mingle, so I'm not sure what kind of access you have to the data. I can envision two scenarios:
*1. API Access to Mingle data
If this is the case, it's going to be very simple to create the new work items programmatically. We do this today between our help-desk system and TFS. It's just a matter of creating a work item, setting the fields, and calling "Save()".
*2. Export access to Mingle data
This is probably an easier solution. Export to Excel (via CSV or custom conversion), and create a TFS list in Excel using the plug-in. Reorder your columns as necessary in the exported data, and copy and paste from the exported data to the TFS data. Click on "Publish" and you're done.
Between the two, I'd recommend the API approach if this is going to be ongoing. If it's going to be a one-time thing, the Excel approach works fine.