That's a pretty wide miss you've got there. I've got fellow brewers that have had issue with hitting gravity numbers using BIAB also, so your volumes on that end could be to blame. Also, what were you figuring for your efficiency and is it accurate to the system you brewed on? You might have just not used enough grain when you converted the recipe. Those are a couple other factors that could have happened. That issue you can figure out later.
Back to your question, there is no "problem" is missing your OG by that much. You end up with a lower ABV beer is all. In order to increase this while it is in the fermenter, you would need to add additional sugars while the beer is at high krausen. That is to say them when the fermentation is really kickin and it looks likes it's about to blow off. One way is to make a small wort again and boil it down, chill it, and then pitch it. You'd want it to be similar to the 1st also. You could do the same thing with pretty much any type of sugar also. But to get back to where you started it might take a bit, and I don't know how to calculate the additions into ABV. Perhaps someone else can help with that. With adding things this way, make sure you mind sanitation.
Personally, I'd let it go man and spice it nicely. The later just seems like a big hassle for 1% of alcohol in the beer really. You'll end up with some between 4-5% anyway, which will be a perfect drinking beer for the colder months. Good luck.