Seven Days to the Rhine is based off / related to Great Escape Games WWII set Iron Cross - and as far as I recall Iron Cross doesn’t have a break mechanism.
But the rules are pretty straight forward - so I don’t think adding a break mechanism would break anything rules wise. Depends how complex you want to make it - I quite like the counter draw mechanism from Battlegroup rules. Each unit as a BP rating, and the sum of these is the BP of the force. There is a bag of counters with BP values on them, ranging from 0 to 5 (I don’t think evenly distributed) along with some special counters. When you loose a unit you draw a counter (but keep it hidden) so your opponent never knows quite how close you are to your break point.