There is no dlc support on Itch. You can't have sub projects.
Create a new project and bundle base item an dlc together for convenience and so people can buy the new project extra.
For your use case there actually might be something. A conditional sale.
https://itch.io/docs/creators/sales#sale-conditions/ownership-requirement
If you have free stuff and just want to give people more that actually paid, you can use
https://itch.io/docs/creators/pricing#purchase-tiers-through-individually-priced...
But those prices cannot be increased, or you will lock out previous supporters.