A solution to train your detector without adding more number of layers when training second time can be by using "CheckpointPath" in trainingOptions. You can use CheckpointPath to save the progress of layers of detector after every "CheckpointFrequency" of "CheckpointFrequencyUnit" (these two are also in trainingOptions)
CheckpointPath can be a location of a folder where you want to save the progress of trained detector.
CheckpointFrequencyUnit can be "epoch" or "iteration"
CheckpointFrequency can be a number, which tells you after how many times again you want to save the progress of trained detector.
In your case if number of epochs are 10 (say) -> then set CheckpointFrequencyUnit to 'epoch' and CheckpointFrequency to 10.
1. Then Load the detector as
load('checkpoint_file_name.mat','net');
2. Resume Training with this detector network as layer graph and the new data you have just as you did the training for the first time.