You can bind a section to a field. It sounds like you are working with a SharePoint List form. So, in the Fields task pane, set the fields to show in advanced view (should be a link toward the bottom of the Fields task pane).
Then, right click a field - any field - and select "More" in the control options - from there you can choose a section.
Alternatively, add a section to your form that is bound to the main SharePointListItem_RW group and change the binding.
You can have as many sections on the form as you like all bound to the same field or group, by the way. I don't know that you'll truly be able to make them "optional" - if you have the user remove the section, it will remove the field it is bound to - but you can add conditional formatting to show hide them.