Use CRM custom field as merge tag in Campagins?

Use CRM custom field as merge tag in Campagins?

When I create a campaign, I'd like to include a custom field from the CRM in the email that's sent.

Is this doable? I see 'crm merge tags' in the campaigns template editor merge tags dropdown, but this doesn't seem to include custom fields. Is there a way to make custom CRM Contact fields appear here to be selected as merge tags?

Or perhaps I'd use campaigns' own user-defined merge tags, but somehow I need to map a CRM Contact custom field to this merge tag?

Thanks