Can I directly add a Common wire to this point on a 24VAC transformer, or will load be too high? (70mA)
If you cannot find a rating on the transformer I would not add the extra load.
With small signal and control transformers it is best to have a “buffer” so you do not over heat it. Small transformers when overloaded will break down much faster than large power transformers as they get hot at first the voltage rolls off and in some cases if they get hot enough and break down the varnish on the wires they can do all kinds of bad things.
When I size control transformers when using relays I usually target a 20+ increase in size so the “pulse” from pulling in the relay doesn’t cause an excessive voltage dip (under sizing with relays can cause chatter and buzzing) chatter is hard on the relay and even worse for the electronics in the circuits think inductive kickback. With the loads you mention I would want a 350ma transformer.
A transformer that is oversized will usually last “forever” but fully load or overload the transformer and the life curves roll off very quickly.