Is it possible to expand the page variable

I want to add a variable to the default.md under the archetypes folder as following

---
title: "{{ replace .Name "-" " " | title }}"
slug: "{{ .Name }}"
date: {{ .Date }}
tags: ["", ""]
draft: true
customtype: true
customname: "1234"
---

On theme file list.html

{{- if .customtype -}}
      {{ .customname }}
{{- end }}

And I got error with infos

execute of template failed: template: _default/list.html:18:11: executing "main" at <.customtype>: can't evaluate field customtype in type page.Page

I checked the help docs, did not find any information about this, if it can be expanded, what is the correct way to do? Thank you.

Thank you so much.