Hi,
We can't really add this by default because it's not really needed by everybody, and nobody want the same thing, etc.
But you can do this with some css, something like:
.hikashop_products .hikashop_product_image img{
-webkit-filter: grayscale(100%);
-moz-filter: grayscale(100%);
-ms-filter: grayscale(100%);
-webkit-filter: grayscale(100%);
-o-filter: grayscale(100%);
}
.hikashop_products .hikashop_product_image img:hover{
-webkit-filter: grayscale(0%);
-moz-filter: grayscale(0%);
-ms-filter: grayscale(0%);
-webkit-filter: grayscale(0%);
-o-filter: grayscale(0%);
}