I'm assuming you were not paid for your part as representing your church. If you want to include these expenses as Donations, you can list your expenses as follows:
1. Go to Federal.
2. Go to Deductions and Credits.
3. Select Charitable Donations.
4. Select Donations to Charity in 2017.
5. Record your Church's Name as the Description
6. Add Mileage and Travel Expenses
7. Head this page with the Conference Name
8. You will put your Total Expenses under Travel Costs with brief description.
