If you want another event then the easiest way is to have the split filter create it for you. You could try something like
code => '
dependency = []
event.get("entries").each { |k, v|
v["dep_name"] = k
dependency << v
}
e = event.to_hash
e.remove("entries")
e["isSpecial"] = true
dependency << e
event.set("dependency", dependency)
'
then after the split
if [dependency][isSpecial] {
ruby {
code => '
event.remove("dependency").each { |k, v|
event.set(k,v)
}
'
}
}