Hello,

Regrettably, this functionality is not available in the upcoming version of Kentico CMS.

However, you can create a new custom form control and use it in any custom table field: Developing form controls

I am not sure if I understand your second question. If you have a fully implemented control which does not require Kentico CMS API or does not need to be inherited from CMSAbstractWebPart and works fine outside Kentico CMS you can User control web part to insert this user control (ASCX) on the page. All you need to specify is the virtual path to the user control.

On the other hand, you can also create a new Kentico CMS web part as it is described in the documentation: Developing web parts

Best regards,
Michal Legen