This information is not specifically for the BTMail app which I have to say is getting very poor reviews and lots of complaints on this forum.
I use the paid for version of the Aquamail app and find it very good there are a load of other very good email apps available for Android.
If you want to place your contacts onto your phone you need to click on the "Contacts" button on the menu bar. Then go to "More" at the top of the list and select "Export" and follow the prompts.
You then need to either need to store the saved file onto your phone or have access to it via your network.
Once you have done that you need to "Import" them to your phone. You open your "Contacts" app then go to "manage contacts" and click on "import/export contacts" . Click "import" and follow the prompts.