Help with WordPress posts query

I have a multisite installation and I have one subsite that contains shared posts that I want all sites to have access to. The shared posts have their own post ID range, like post ID > 1000000000... I want to be able to modify the WordPress query on posts so that these shared posts are added to any subsite post query.

So for instance site A has posts 1, 2, 3 ... and site B has posts 10, 11, 12 and the shared site has posts 1000000001, 1000000002, etc. but if you do a search of posts for site A you get 1, 2, 3, 1000000001, 1000000002, etc. even though the posts are on different sites.

I want shared site post will be included in any subsites post query. How can I achieve this?