There seems to be no way to simply place a red joint dot on a wire or a crossover of wires. I have tried copying and pasting a dot and several different approaches to adding a short segment of wire to an existing wire but none of them leave a join dot behind. IIRC copy and paste used to work but that has probably been removed to help prevent accidental joins being created. It is possible to join wires by giving the wires to be joined the same net name but this can lead to the accidental creation of multiple netnames on the same net and can easily create accidental joins. Whilst these can be checked for by running the Schematic Design Manager, it is extra work and extra risk. The safest solution seems to be delete one or more segments of wire and then redraw to create the join. Please note however that it  is good practice to join wires withe staggered joints like this: