Convert HEX to RGBA in Stylus

I have some HEX colour values in Stylus similar to the following

$my-background ?= #123456
$my-foreground ?= #ABCDEF

and would like to use them in rgba values for opacity, so that

.my-class
    background rgba($my-background, .5)
    foreground rgba($my-foreground, .5)

or an alternative syntax is compiled into CSS as

.my-class {
    background rgba(18, 52, 86, .5);
    foreground rgba(171, 205, 239, .5);
}

Is there a quick and easy way to use HEX colour values in rgba using Stylus or a plugin for Stylus (such as nib)?