Not too sure if there is already such feature in Balsqmiq.
Anyway, will be good to have direct Quick Add at mouse cursor as another way to add using the toolbar during layout .
I find out less distracting to add this way as our eyes already at the editing area, thus less likely to disrupt the “flow”.
This Quick Add feature could be more intelligent as to show only 4 commonly used items based what is already been used in the document and initial letters. It can be activated at like shortcut key (eg A key)