And then wait 4 to 6 weeks for the "new" bar to dry and harden. If you are trying to get a nice hard bar it will take some serious patience waiting.Is your tea tree 100% Tea Tree Essential Oil? If they are calling it "tea tree oil" it is probably a tiny amount of tea tree essential oil in a carrier oil such as sweet almond.
If you want a solid bar, I would recommend grating the soap very fine, then adding a small amount of water - say 1/2 - 1 teaspoon - stir it well, then let the shreds sit overnight. Since it is such a small quantity, I think rebatching in an oven bag or in a boiling bag might be best.
Then after your soap is liquid-y, put it in a bowl, add your tea tree, stir well and put in a mold.