As you have found, the Questions page is a one way street, where only clients can do the asking. That way, everyone can quickly access what clients have asked and how you've answered. But, what should you do when you have questions for your clients?
The vast majority of questions a builder has for clients are related to decisions, such as "Do you want to add recessed lighting," or "Are you ok with the hardwood upgrade pricing?" Since the Specs & Selections is the hub for all decisions, simply add a new selection item, add a choice to an existing selection, or add a comment to an item to pose these questions.
For ancillary questions that are not related to project decisions, like "Can you meet on Tuesday at 2pm?" you can use the Messages page.