I need to include a few contra revenue accounts that have 1099 contractor payments in them in our 1099s but the accounts are not listed in the 1099 process to select.

contra revenue for vendor payments?

vendor (contractor) payments are an expense, on the tax form they are listed as non employee compensation, in the expense section.

But if you insist, in 1099 mapping, block 7, select multiple accounts and check of all that apply.  I have never looked to see if income account are included in that selection, I doubt it though since it is rightly an expense.