BETTER FORMATTED COMMENT:
It requires downloading a browser extension, but works rather nicely in Chrome. (Possibly Firefox as well, though I haven't tested it.)
First, get ColorZilla: http://www.colorzilla.com/ An eyedropper icon should appear in the top right corner of your browser.
Second, Use the eye dropper tool to pick whatever color you want in your browser.
Third, click "Color Picker" from the ColorZilla dropdown menu, then in the window that pops up, 'Ctrl+C' the numbers in the parenthesis next to RGB.
Fourth, 'left click' THEN 'right click' any color in your own active drawing swatch and click 'inspect element' from the drop menu.
Fifth, in the panel that pops up, double click the highlighted line so it becomes editable. Then, highlight the numbers in the parenthesis after "background color" and hit 'Ctrl+V'.
Click out of the panel, and your new color should appear in the swatch! (Click the color swatch again to use it.)
As a tip, if you want to free up swatch spaces, dedicate a layer of your drawing to tracking your imported colors, and just use the regular eyedrop tool whenever you want to reselect them.