1. Mohamed already replied to you about that
I think that you'll have to use CSV import and directly put the URL of your images in the "images" column then HikaShop will download them automatically to your website in order to attach them to your products.
2. You would have to create first a custom field of the table "product" in order to enter the URL of the image for each product.
Then, you would have to edit each view file of the view "product" via the menu Display->Views and replace the image displaying block of code by some simple code like that:
echo '<img src="'.$this->row->custom_field_column_name.'" />';
Where custom_field_column_name is the column name of your custom field. Of course, you can add a width/height in that img tag to force the size of the image in order to suits your needs.
Here is an example of such code that you want to replace:
echo $this->image->display(@$this->row->file_path,false,$this->escape($this->row->file_name), '' , '' , $this->image->main_thumbnail_x, $this->image->main_thumbnail_y);
That's taken from the listing_img_title view file.