Yeah mix your additives in a bucket with hose pressure. Add this liquid to mixer....then half the sand ....all the cement ......and rest of sand. Always remember...you can put more water in but you can`t take it out
The practice of mixing sand and cement together and then adding water goes back to the days of "hand mix" on two 8x4 spotboards. We dry mixed the sand/cement twice, then spread it out in a ring to put the hole in the middle for the water then pulled the "walls" of dry mix into the puddle etc etc.
This method was used for smaller jobs off site until electric mixers became generally available. The mortar was never quite as good though.
I`m sure more than me remember all this.......hopefully!
Alex