Can I use "ml" as a product volume ?

I use HikaShop Business: 2.2.2 in Joomla 3.1 and would like to add "ml" as a volume because we mainly sell bottles. It should not be a (alternative) variant because every product is different. We don't have a specific bottle in different volumes.

Can this be done? Can I add "ml" to the weight-list or volume-list ?

Thanks.

You can't. However, your can change their translations.
If you add the line:
KG="ml"
to your translation override, you will have ml displayed instead of KG for the weight of the product (and a similar modification is possible for the product dimensions).