Greetings,
If I understand your question, you want to enter ingredients that are not included in BS2 and they are not available as an Add-ON, is that correct? All you need to do (in BS2 Desktop) is click on Ingredients, then Miscellaneous. After the list is populated, click on Add Miscellaneous in the top left corner on the ribbon. A dialog box pops up that will allow you to enter your new ingredient.
Now, as I understand, you want to have your particular ingredients grouped together. I don't believe BS2 allows you to create a particular group. So, what you can do is simply add a special name to your ingredients to make them appear as a "group". In other words, your group name may be: "Salts". Now simply start each ingredient you want in that group with the name Salts - <name>. Then when you're developing a recipe, click the Miscellaneous button, and scroll to your "Group". Or search for your "Group" using your group Name - Salts - and all your ingredients will appear.
I hope this answers your question.