Ivo,
Thanks for the suggestion. I've added it to our feature tracker as something to look into in the future. However, while a normal 'website' field in many extensions may not accept multiple websites, many forum components actually do accept multiple websites as the website field can be pulled into a signature (text area) or similar type field which can hold multiple values of text.
Hope that explains the reasoning behind it, but we can try to make the logic a little smarter when you're importing into specific fields.
Thanks,
Alex