For wireframes / mockups we are currently using Axure.
It would be great to be able to use that instead of the Aha! build-in tool.
When I think about it, your tool is rather a diagramming tool like LucidChart. To call it "mockup" is misleading in my opinion.
Thank you for the request. You should know that the Aha! REST API is available and this is actually what we leverage when building in-house integrations.
At this time, we do not have near-term plans to build this specific integration based on current priorities and historical feedback. However, we would encourage you to use the API to build a custom application for your organization's use.
Seconding integration for Axure... the built-in mockup tool (which looks like draw.io if I'm not mistaken) is good for orgs that already don't use something else, but definitely isn't as full-featured as Axure.