This tip used to be part of Quark's tips and tricks showed on their site. I cannot find it back.
This said, and stated in the pointed article, you won't be able to create a new color out of that unless you manually copy the values.
If you search Forum archives you'd find an applescript allowing you to pick a color from Adobe Photoshop (that is the foreground color actually, so the color you've just picked from an image) and create it straight into your Quark document.
Also another one allows you to pick several colors inside of PS and then creates those ones into your Quark doc.