Altium: split planes on component layer
Altium doesn't provide any capability to make one area of a layer negative while the other is positive. This is related to how the boards are actually made.

In manufacturing, the design artwork is plotted on a film as a negative (dark where the CAD has a feature, clear elsewhere), and the board can be coated with either negative or positive photoresist before exposure and etching. If positive photoresist is used then the areas of resist that get exposed to light can be washed away, allowing etchant to remove the copper beneath them. With negative resist, the unexposed areas are washed away and the copper beneath them is removed by the etchant. It isn't practical to coat one area of the board with negative resist and one area with positive resist.



(Image shows a semiconductor processing example, but the principle is the same when the substrate is FR4 and the etched layer is copper)

Positive (signal) and negative (plane) layers in Altium map directly to using the two types of photoresist in the manufacturing process.

I have seen old designs where a revision was made by adding a 2nd film that the board would be exposed to after processing the original film, effectively combining two films onto one layer. However you wouldn't want to do this if you can avoid it because it adds processing steps and cost.


  I can do it with polygon pours but modification of a polygon sometimes involves the need to modify other polygon.


Any version of Altium less than 10 years old makes it quite easy to manage polygons, including changing their shapes and specifying pour order to control the effect when two polygons overlap. I strongly recommend using these features rather than trying to combine two CAD layers onto one layer in the final product.