[Adar12]

1) Is it possible to import all posts in LJ community to my BuddyPress group? And with comments tree?
2) Is it possible not only import but regular updating this BuddyPress group from LJ community new writings? By using RSS-import plugins I suppose?
3) Is there any solutions for crossposting between LJ and BuddyPress blog (and group)? And between separate WordPress blog and BuddyPress one?

[Virtuali]

What is LJ? If it’s another group or forum software, you will need to search for a LJ to wordpress bridge. Otherwise, there is no other way to import, unless there is a feature with the LJ software.

[Adar12]

LJ = LiveJournal[.com]

[@mercime]

1) You could import/export all posts and comments from LJ to WordPress site. https://codex.wordpress.org/Importing_Content#LiveJournal. Didn’t check what WP version the script was compatible with, but you can download WP versions from this page
and upgrade to later version later.
https://wordpress.org/download/release-archive/

Currently, there are no scripts to convert these into a BuddyPress group setting. You might have to hire someone to consult with or to implement that as well as import users automagically as BP/WP users along the way.
http://jobs.wordpress.net/

#2 and #3. Haven’t seen any indication that this can be done except probably with RSS imports/exports. I suggest you clear up #1 first before moving along these questions.