Dealing with long sequence labeling
First of all, don't use classification accuracy as a metric. Use precision, recall or F-score. They are best suited for multiclass unbalanced datasets.

Secondly, if you want to enrich your dataset with synthetic points of the minority class, a common way is to use the SMOTE algorithm.

Another way of creating more synthetic samples of your classes would be to use Generative Adversarial Networks (GANs). You will train a Generator and a Discriminator network based on the samples that you have. See this for further info.