As you didn't provide either your module code or the code that you've tried to generate the sku, i can provide you with a few ways how you could generate a SKU for the new products that you import through your Mass product Update Module.
- Create a custom module and observe either of these events
catalog_product_save_before
or catalog_product_attribute_update_before
there are also other events like, which you can try and use the one suits your requirements.
catalog_product_save_after
catalog_product_save_commit_after
catalog_product_attribute_update_after
Get the product(s) data from the Event Observer Model that you should create
Check whether its a new product
In case of new products, generate a SKU and then set the SKU and save the product.
Hope this helps !
Also read these Magento SE threads below to get the proper understanding of these events and Magento ORM
How can I differentiate between product creation and saving existing products?
https://magento.stackexchange.com/questions/120004/difference-between-catalog-product-save-after-and-catalog-product-save-commit-af
How prevent a model data save using _save_before event
Will $product->save() trigger catalog_product_save_after?
catalog_product_save_after Event for Massaction
BTW if you want to get help from others on this platform, you need to provide the code that you've tried.