How to extend SCSS custom property?

Question

Sass

There is a custom property rgb, inheriting the css properties of which, I have to create a new custom property with a certain transparency. How can this be done correctly? - initial custom property. It should look like this: - a new custom property with transparency. It is necessary to expand the original.

$bg_main: rbg(32 , 32 , 32);


$custom_bg_main: rgba($bg_main , 0.5);