Hi, you can do this using the Shopify Import/Export functions from the Products page (buttons on the upper right of the page). Here are the steps:
1. Export your inventory as a .csv file. You can typically open this file in Excel or a database program like Access, or FileMaker (Mac or Windows) or Numbers on the Mac.
2. Create your SKUs using whatever combination of string functions that you wish, or manually.
3. Import your file back into Shopify using the "overwrite" option, to replace data for existing products. This will load the SKUs that you created in step 2, and leave the rest of your product information intact.
Suggestions:
Try this with just a couple of items from your inventory before doing it on your entire inventory.