I don't use User Manager at all, but if you can't find a native way of doing this, you can easily generate the CLI commands to add accounts on another machine and just paste them in.
Below a quick and very dirty sample as a shell script (requires seq and openssl, everything else should be standard):
#!/bin/sh
if [ -z "$1" ]
then
echo "Usage: generate_pins.sh [length of PIN] [number of PINS]"
exit 1
fi
LENGTH="$1"
if [ -z "$2" ]
then
echo "Usage: generate_pins.sh [length of PIN] [number of PINS]"
exit 1
fi
NUMBER="$2"
for i in `seq 1 "$NUMBER"`
do
PIN=`head -c "$LENGTH" /dev/urandom | openssl base64 | tr '[/=+]' '[ZzZ'] | head -c "$LENGTH"`
echo "/tools user-manager users add username=$PIN password=\"\""
done
And a sample run:
sh-3.00# sh generate_pins.sh 6 5
/tools user-manager users add username=YSPKjQ password=""
/tools user-manager users add username=9PX38J password=""
/tools user-manager users add username=hfPvae password=""
/tools user-manager users add username=8wMtj7 password=""
/tools user-manager users add username=xJQMJR password=""
That line may not be a valid command in RouterOS to generate a User Manager user (as I said I don't use it and don't have it installed anywhere), but it should be easy enough to adjust the script to output valid commands.