unfortunately, there is not a perfect solution to this problem (yet). Even if you were allowed to input the quantity by keyboard, units of 1 g would not provide a good solution because the foodsoft enforces rounding of the prices after two decimal places.
A good place to discuss these kind of issues is the foodsoft’s issue tracker on github. Somewhat related is issue #474. You may consider to create a new issue there in order to “request” a feature to be implemented. However, there has not been much development activity on the foodsoft recently. If you (or someone else from your foodcoop) are a programmer then you can help to improve the foodsoft yourself. Otherwise we have to wait for someone else to work on that.