#!/bin/bash
s='line...
*action.acroread: {APPMGR_acroread("blah.pdf");}
another line...'
sed '/^\(\*action\.acroread[^{]*\)\(.*\)/ s||\1/* \2 */|' <<< "$s"
IyEvYmluL2Jhc2gKcz0nbGluZS4uLgoqYWN0aW9uLmFjcm9yZWFkOiB7QVBQTUdSX2Fjcm9yZWFkKCJibGFoLnBkZiIpO30KYW5vdGhlciBsaW5lLi4uJwpzZWQgJy9eXChcKmFjdGlvblwuYWNyb3JlYWRbXntdKlwpXCguKlwpLyBzfHxcMS8qIFwyICovfCcgPDw8ICIkcyIK