Besides the units the are already preset, users can add their own set of custom units to be converted.
IMPORTANT: When setting-up custom group of units click SAVE after creating the group and the Primary unit. After clicking SAVE, you will be given the option to add additional unit in the newly created Group.
To make this extension even better it might be an option to make the field rewritable. For the moment i have to delete a field if i mistyped a word...
If these issues are solved i am totally happy!