Add something like 'Use current desktop wallpaper' into 'Wallpaper Type:' dropdown menu in 'Wallpaper' tab. For better coherence between system's elements.
I'm totally honest: this is too difficult to implement in a way which would work reliably. The main problem is that there is not a "current wallpaper" in Plasma. There is a current wallpaper per screen per activity. The lock screen neither knows about activities nor about multiple screens. This makes it a rather complex feature to implement. The engineering effort is too high compared to the benefit.