The compatibility between Meta Box and WP All Import (WPAI) is maintained on WPAI side, so I suggest you contact their support to get further assistance. I also will inform our development team to cooperate with their team to support import value to existing field value.
Thanks that'll be very nice have it from WPAI, but they replyed me they could develop it if enough people asking this feature⦠But I think it's very important to be really usefull.