How to add product variations to woocommerce custom product type?

Question

WordPress

Good afternoon. The woocommerce site has a custom product type (essentially an analogue of a simple product). It is necessary to stick to the specified type of product the ability to add product sizes and quantities for each size of this product. So that the output is the same as in variable products.
For example: T-shirts are available:
size 42 - 5 pcs,
size 43 - 3 pcs,
size 44 - 2 pcs.
The buyer purchases 1 T-shirt size 42, remains:
42 size - 4 pcs,
43 size - 3 pcs,
44 size - 2 pcs.
etc.
I didn't find suitable plugins. Is it possible to file this through function.php or in some other way?