To answer this I have to presume a couple things. With that, I'm going to use the scenario that the schedule for week 2 (and beyond) was changed to include the two new teams, and that they bowled whatever other teams were on the revised week 2 schedule.
Further, this scenario would mean that the two new teams (numbers 17 and 18) would bowl each other in week one and the post-bowl was just that.
The schedule should be re-loaded from week 1 anyway If the makeup is going into week 1, otherwise BLS will not consider the league to have 18 teams in which to place scores, award points, etc.
As long as the league uses book averages for the first x games (say, 9 games) or first x weeks (say, 3 weeks), the bowlers' entering averages would be the same for weeks 1 and 2. So, you could easily insert the scores while returned in week 1 (when you go back to week 1, you have to tell BLS why you are going to week 1...the option including corrections/makeup scores is the one to select).
The only case in which you'd have any issue with this would be if you had a bowler with no previous average and their scores from their first night of bowling would determine that average. If they bowled the makeup before week 2 was bowled, no problem. If the post bowl was done after week 2, then for those bowlers, you'd have to override the average for those bowlers when you go back to week 1.
If a significant number of bowlers on these two new teams have no averages, then you'd be better off entering the makeup scores in week 2 (indicate that they were bowled for week 1), then go back to week 1 to let the scores and averages be "picked up" by BLS and points awarded accordingly.
I know I've written a lot...but you asked a question with limited information. Rather than answer the question with another question, I went off a plausible scenario and worked with that, explaining why it could work as you suggest with your question, and then giving a couple situations where it might have to be handled differently.
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