#!/bin/bash
s='start
line1
line2
line3
end'
sed '/start/{:a;N;/end/!s/$/</;/end/!ba;}' <<< "$s"
#sed -e '/start/,/end/{/start/{n;s/$/</;n};/end/!d}' <<< "$s"
IyEvYmluL2Jhc2gKcz0nc3RhcnQKbGluZTEKbGluZTIKbGluZTMKZW5kJwpzZWQgJy9zdGFydC97OmE7TjsvZW5kLyFzLyQvPC87L2VuZC8hYmE7fScgPDw8ICIkcyIKI3NlZCAtZSAnL3N0YXJ0LywvZW5kL3svc3RhcnQve247cy8kLzwvO259Oy9lbmQvIWR9JyA8PDwgIiRzIg==