Apache OpenOffice Community Forum

Would it be possible to somehow implement this feature with an extension? How difficult would it be? Where can I start?

Welcome to the Forums.
I'm sorry to say that I don't understand what it is you're actually wanting.
A spreadsheet can be considered to be a table, as it has two dimensions, like a tabletop (please, don't get literal about a tabletop actually having 3 dimensions).
You can make lookup tables for use in a spreadsheet, but I don't know if that's what you mean by a table.
Some people also refer to graphs as tables, which they can be, but in Calc they are referred to as graphs.
You state that this is a feature, yet it's one, if you're referring to it correctly, that I've not encountered before. Where/how did you come across it?

I saw this feature in Numbers. You get a sheet where you can put different elements like texts, images, etc. separetley from the table, and tables are elements of the sheets too, and you can have many in a single sheet.

I've never used Numbers, but have seen it referenced in regards to spreadsheets.
While all spreadsheets share very basic operations, eg: =1+2, I've never encountered two spreadsheet programs that have exactly the same functions, so while this may be in Numbers, you cannot presume that it exists in OpenOffice Calc.
Someone with more knowledge of Numbers may be able to clarify on this.

Any rectangle of cells can be treated as a table.