I don't use pose morph or set driver, but as I understand SD it just makes an XPresso tag with a range mapper with 1 to 1 correspondence. So my guess is to make an XP window, drag each pose tag in, for the driver pose drag the sliders to the RHS to make a port and ditto LHS of the driven pose. Then connect with wires. If both are 0-100% there shouldn't be a problem and a rangemapper won't be required.
[ RHS of node is output, LHS is input ]
might work...