PowerShell Create Hashtable if it does not exist and adds values

Currently, I'm taking a manually created hash table so I can browse more times

$ aceList = @ {
"Domain  jdoe" = "Edit, Send, GetPassword"
"Domain  ssmith" = "Edit, Send, GetPassword"
"Domain  msmith" = "Submit"
}

However, that does not allow me to summarize it further.

Ideally, what I would like is something like this, without having to adjust the $ acl = @ {} outside the function?

Set-HashTable function {
param (
[String] $ Identity,
[String] $ Access,
[Hashtable] $ ACL
)
$ ACL.Add ($ Identity, $ Access)
back $ ACL
}

$ acl = @ {}
$ acl = Set-ACL -Identity "Domain  jdoe" -Access "Edit, Send, GetPassword" -ACL $ acl
$ acl = Set-ACL -Identity "Domain  ssmith" -Access "Change, Send, GetPassword" -ACL $ acl
$ acl = Set-ACL -Identity "Domain  msmith" -Access "Submit" -ACL $ acl