How to inject a post within a loop

I am assuming you are doing something like this:

if (have_posts()) {
  while (have_posts()) {
    the_post();
    if(x == j){ //inject post here }
  }
}

Injecting a post inside the Loop like that will cause the post count for that page to differ from the posts_per_page value, but it will not break pagination (which seems to be your primary question). Pagination operates on the main query (usually) and that will not be altered by your insertion.

I don’t know how you are retrieving the additional post data but if you are not careful you can break things with that.