Importing nodes from custom XML feed: how do I convert some XML fields into entity references? Here's the problem:

I'm migrating a Wordpress website to Drupal. It's a gaming portal with a highly customized Wordpress with custom fields, so we decided to generate some XML dumps from Wordpress, then use Feeds module to import that. There are two different "content types", games and articles about those games (strategy guides, tricks, so on). We have two content types, so we generate two XML files. All good so far.

The problem is that those two content types are related. To be precise, the "article" content type is always referred to a game. In Drupal terms, "article" contains an entity reference fields that points to a node of type "game".

Where's the problem?
Example: we have an article entitled: "8 tricks for Clash of Clans". This article refers to game "Clash of Clans". The problem is: in our XML we have "Clash of Clans", written in words. I want Feeds module to those words to Node IDs, then save those Node IDs as node references.

I want to do the same with some Taxonomy Terms.

Makes sense?
Maybe a bit late...

Do you know the feeds_tamper module?

But - I think you won't get around writing your own feeds_tamper plugin.
You have to parse your input field and search for node IDs, then replace it with the ID, as you wrote. This can not be done with feeds_tamper standard plugins, because even with REGEX-replace you don't have access the node IDs.