In that case, just put the createfile command after the prefetch and before the copying of this file over, like this:
prefetch RoamingClient_MAC.pkg ????? prefetch stuff ????
delete __createfile
delete __Download/OrgInfo.plist
createfile until _END_OF_FILE_
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>APIFingerprint</key>
<string>123456789</string>
<key>APIOrganizationID</key>
<string>98765432</string>
<key>APIUserID</key>
<string>11111132</string>
<key>InstallMenubar</key>
<true/>
</dict>
</plist>
_END_OF_FILE_
copy __createfile __Download/OrgInfo.plist
folder create "/Library/Application Support/OpenDNS Roaming Client"
delete "/Library/Application Support/OpenDNS Roaming Client/OrgInfo.plist"
copy __Download/OrgInfo.plist "/Library/Application Support/OpenDNS Roaming Client/OrgInfo.plist"
wait /usr/sbin/installer -pkg __Download/RoamingClient_MAC.pkg -target /