How to meet code when the outdoor light is separated from the electrical box by a wooden spacer
Yes, there is such a thing as a weatherproof extension ring

While you can't just extend the wires out beyond the box to the fixture (what if a splice failed and tried to light things on fire?), the good news for your situation is that weatherproof extension rings do exist! Ask your local supply house for a Hubbell (Bell/Raco) 5363-0; this will give you 1.5" of extra box depth, which should be just enough to deal with your spacer board.