In the UI wireframe designer, is it possible to create a data grid with combo boxes inside the cells?
As a feature this appears to be planned but low-priority for Balsamiq (Balsamiq makes the software, we just have a plugin to let us use it). You can click the +1 to like the suggestion to show support for the feature.
As a quick and dirty solution, you can just layer comboboxes on top of a datagrid:
Use the "bring to top" layering item to make sure the combos are on top (they should automatically appear as the top item as you add them though) and you can sit them over table cells. Since ultimately what people on the site see are images of the wireframe anyway, the end result isn't that different from a more proper implimentation.