Need an extra pin to control my driver board
Rather than try to use the same pins as both analog inputs part of the time and digital output(s), I would do the following:

Add an 8-channel analog mux to the driver board, such as the HEF4051B.  The output of the mux will then go into one of the analog input lines currently going to the MCU board.  Tie the enable lead \$\mathsf{\small \overline{\text{E}}} \$ to ground (always enabled).

Reconfigure the remaining seven analog inputs as digital.

Use three of the seven digital lines to select the 1 of 8 channels for the mux.

You now have four digital lines available between the MCU board and driver board.