reference other feed and/or node field from filter

-reference another node field from the filters

You may now use in your filter function FeedImport::$current object which is a reference to current importing item (just added it to dev).

reference another feed value from the filters

I don't know what you want to do here.
You want to use some info from previous processed items? If so, this is somehow not possible because if you have a huge feed previous items will be deleted after save to free memory.

Please use an example so we can find a solution.

Let's assume that we already know the text and url and we want to create a link.

function create_html_link($field) {
  $current = &FeedImport::$current;
  return l($current->text, $current->url);
}

I have field "Link", and two elements 'url' and 'title'(in xml two strings). how I can use FeedImport::$current?

There is a simple example in #3 --a function used as filter. You cannot use FeedImport::$current in UI.
Assuming that Link field has two columns: "text" & "href" and you already extracted those values in your current item, you can write a filter function which you'll use in UI.

function MY_MODULE_get_curret_link() {
    $current = &FeedImport::$current;
    return (object) array(
      'text' => $current->title, //assumes that you alread extracted title
      'href' => $current->location, //same for this
    );
}

Anyway, please give me more info about what you want to do (so I can help you).

I have field link provided by module http://drupal.org/project/link
and has the following structure
field_post_link array(
und array(
0 array(
url => http://blogurl.com,
title => blog name)))
in xml files I has following structure

<blogName>blog name</blogName>
<blogUrl>http://blogurl.com</blogUrl>

how I can import "title" and "url" together?
sorry for my English)

xpath sould be: blogName | blogUrl