Hi Miguel1
Yes, you can do this. Export your products using the Products entity and in 'product_type' column, just change 'Product Part' by 'simple. Then, import the spreadsheet using the UPDATE options.
Also, I'm not sure if this is what you want to achieve, but if you want that products parts to be displayed in Stock Central, you can enable this feature in ATUM Settings > Products Levels > Display sellable BOMS (In case your product parts are sellable).
Best Regards,
Andrea.