Custom transliteration

I think this is a question for Pathauto, not Transliteration.

It doesn't look like there's a straightforward way to do this natively in Pathauto without changing the available transformations for punctuation. The simplest way to do this is probably with hook_pathauto_alias_alter(). Ensure that Ampersands are set to "No action (do not replace)" at /admin/config/search/path/settings, then add a hook_pathauto_alias_alter() in a custom module like this:

function mymodule_pathauto_alias_alter(&$alias, array &$context) {
  $alias = str_replace('&', 'and', $alias);
}

This will then replace any ampersands in your URL with the word 'and'.

I'm going to mark this as Fixed since this should solve your issue, but if you have further questions please set it back to Active. If you'd like this to be a fully-fledged feature of the module, then consider filing this as a Feature request.