We are planning to establish a product database on wordpress via custom post types. and import those posts to discourse wia WP Discourse plugin.
Since each wp product page will have several custom fields, we would like to map them to discourse custom fields for each corresponding topic. How could that be achieved?
You should handle this via a proper migration on the server.
I wouldn’t use WP Discourse, or the Custom Wizard Plugin for this (which I know is also in the mix here). Any mixture of those solutions is going to be more trouble than it’s worth for a bulk operation like this, particularly one involving custom fields. You will spend more time and effort for a worse result trying to use the plugins for this.