I think you can skip sending the original confirmation mail in your admin action. Simple create your own mailer action and send your desired mail.
Maybe it is also possible to create a normal confirmation mail, but modify the mail output.
bundle exec rails generate devise:views
And after that modify the text in "app/views/devise/mailer/confirmation_instructions.html.erb
<% if @sent_by_admin == true %>
...admin mailer text...
<% else %>
...normal mailer text...
<% end %>
@sent_by_admin could be a helper variable, which is passed to the devise confirmation mailer.