When it comes to fixing a wood nipple into a piece of wood it can be made easier if you add a nut to the nipple first.
So the best way is to add a nut to the nipple so it sits at the bottom of the fine 10mm thread.
Then add the lampholder to the nipple right up to the nut.
Then screw the nipple into the wood using the lampholder.
However make sure there is not too much force being put on the lampholder.
If you are fitting to hardwood it will be harder to insert the nipple.
Once inserted you should be able to remove the lampholder and then the nut.
Add the base of the lampholder back onto the nipple and wire.