I'm not sure of an external box that can accept RGBcvS scart or YUV Component, but the Sweetspot capture card should do what you want. Have a look
Here for more details.
As for an external converter, well I think the
Canopus TwinPact-100 may do what you want. its specs say it can input RGB, but not which versions of RGB. It has a VGA input connector and this is normally RGBHV which is not directly compatable with RGBcvS (scart RGB), but you may be able to set the version. You will need to check this out if interested.
I've had a quick look at the
Director's Cut Scart web site and there appears to be no mention of the video input signals it can accept. I would therefore think it would just be composite video (but I may well be wrong).
Mark.