You can't use the syntax you're wanting.
You could use jinja to shell out and list any files in that directory. That would probably look something like this:
Contents of /srv/pillar/top.sls
:
{% user_pillars = salt['cmd.run']('ls /srv/pillar/user*').split() %}
prod:
'*':
{% for dir in user_pillars %}
- {{ dir }}
{% endfor %}
I haven't tested this exact code, but it should be close to that.