You can, but you don't really need to. Azure AD B2C is a separate resource from your main Azure AD and your user directories don't intermingle.
You can create an Azure AD B2C tenant that uses your existing Azure AD tenant as the 'home tenant' that has a subscription. Take a look at this documentation, it should explain the ideal approach to setting up your B2C tenant: https://learn.microsoft.com/en-us/azure/active-directory-b2c/tutorial-create-tenant