i need it for questbook-like application, but i don want to show them on page, just in the admin tab-comments...

Sorry, I do not think I understand your request.

If it is to accept comments but not display them on the gallery page you would need to remove the while (next_comment()) loop from your page.

i know that, but is it possible to see the comments made on custompage in admin interface?

If they are posted to the database they will show on the admin pages. But remember that there must be an object associated with the comment. Your best approach would be to use zenpage and have one of its pages as your guestbook.

and is it possible to somehow activate the comments on pages without zenpage? currently its not working.

Comments must be associated with one of: an album, an image, a zenpage article (news), or a zenpage page. So you could also have an album for your guest page.

I'm using a empty, unpublished album as a guestbook, just as sbillard suggested. In the example theme you will find how to add comments to albums.php if you haven't done that yet.