Summarized:
1. Link your PIM Tab field to your Main Tab
2. Import into your Main Tab
3. The value from your Main Tab is now transferred to your PIM Tab field
Explanation:
At this moment, it is not possible to import directly into a PIM Tab, but it is possible to link fields from the Main Tab to your PIM Tab field. You can see how to do that below. (Read the further instructions below the picture).
After you have linked the field from your Main Tab to your PIM Tab field, you can perform a regular import into your Main Tab. If you have linked the field correctly, the value from your Main Tab will be automatically transferred.