@awginger

Something like this may help you?

#!/bin/bash





jssURL="https://xxxxx.jamfcloud.com"



## DO NOT EDIT BELOW THIS LINE ##



echo "Please enter your JSS username:"

read jssUser



echo "Please enter your JSS password:"

read -s jssPass



usersToGetAtOneTime="10000"



# Make sure xmlstarlet is installed

if ! command -v xmlstarlet > /dev/null ; then

    echo "You must install xmlstarlet before using this script."

    echo "Try "brew install xmlstarlet""

    exit 1

fi



# Get list of users

echo "Downloading list of user IDs..."

while read -r line; do

    ids+=("$line")

done <<< "$(curl -X GET -k -s -u "$jssUser:$jssPass" "$jssURL/JSSResource/users" | xmlstarlet sel -t -v '/users/user/id')"



# Find users with roster_managed_apple_id in

echo "Gathering User Information"

count="$usersToGetAtOneTime"

for id in "${ids[@]}"; do

    managedAppleID=$(curl -X GET -k -s -u "$jssUser:$jssPass" "$jssURL/JSSResource/users/id/$id" | xmlstarlet sel -t -v '/user/roster_managed_apple_id')



# Update Managed Apple ID if Roster Managed Apple ID Exists   

if [ -z "$managedAppleID" ]

then

    echo "No Managed Apple ID, moving on"

else

    echo "$managedAppleID"

    curl -X PUT -H "Accept: application/xml" -H "Content-type: application/xml" -k -u "$jssUser:$jssPass" -d "<user><managed_apple_id>$managedAppleID</managed_apple_id></user>" "$jssURL/JSSResource/users/id/$id"

    ((count--))

fi

done



exit 0

If you are move comfortable manipulating CSVs than scripting, you can check out The MUT.
There is a good How To Video on their page

Thanks @chris.reynolds that worked a treat! Also thanks @shaquir, I will check that out!