I understand that you would like to add an additional email address (iCloud or any other email account) so that it can be used with iMessages. NOTE: You can cannot use SMS text messages with email addresses. You can use an email address only with iMessages.
You can add and remove email addresses associated with your Apple ID by opening Settings ➡️ Apple ID (Your Name at Top) ➡️ Sign in & Security ➡️ Edit: Here, you can add or remove an email address, trusted phone number etc.
Once you have made those edits to your Apple ID, then those email addresses will be available to use with iMessages in Settings ➡️ Messages ➡️ Send & Receive.
Axel F.