wget -O metallb-values.yml https://raw.githubusercontent.com/bitnami/charts/master/bitnami/metallb/values.yaml
cat > /tmp/0 <<EOF 
configInline:
  # The address-pools section lists the IP addresses that MetalLB is
  # allowed to allocate, along with settings for how to advertise
  # those addresses over BGP once assigned. You can have as many
  # address pools as you want.
  address-pools:
  - # A name for the address pool. Services can request allocation
    # from a specific address pool using this name, by listing this
    # name under the 'metallb.universe.tf/address-pool' annotation.
    name: generic-cluster-pool
    # Protocol can be used to select how the announcement is done.
    # Supported values are bgp and layer2.
    protocol: layer2
    # A list of IP address ranges over which MetalLB has
    # authority. You can list multiple ranges in a single pool, they
    # will all share the same settings. Each range can be either a
    # CIDR prefix, or an explicit start-end range of IPs.
    addresses:
    - 192.xxx.xxx.xx1-192.xxx.xxx.xx5
EOF
sed -i -e "/configInline: {}/r /tmp/0" -e "//d" metallb-values.yml

echo ${DATA} > temp.txt    
sed -i -e "/_data_/r temp.txt" -e "//d" mail.tpl

DATA="a
b
c"

ESCAPED=$(echo "${DATA}" | sed '$!s@$@\\@g')
echo "${ESCAPED}" 
a\
b\
c

sed "s/pattern/${ESCAPED}/" file

DATA="$(cat whatever)"
ESCAPED_DATA="$(echo "${DATA}" | sed ':a;N;$!ba;s/\n/\\n/g' | sed 's/\$/\\$/g')"

Then you can use ${ESCAPED_DATA} in sed:

cat input | sed 's/one liner/'"${ESCAPED_DATA}"'/' > output 

$ echo test | sed 's/test/line\
> line'
line
line