Microsoft’s add-in program is a key to its efforts to make Office a development platform in addition to a suite of productivity applications.

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SAN FRANCISCO — What is Microsoft’s platform?

The easy answer is Windows, the plumbing that makes most personal computers tick and Microsoft for decades used as a lever to get people hooked on its Web browser and other software.

At the Redmond company’s Build developer conference in San Francisco last week, Chief Executive Satya Nadella made the case that the company’s Office programs like Word and Outlook are also building blocks on which developers can create programs.

The add-in concept isn’t exactly new. Office 2013 was released with support for programs built by others. But Microsoft, which has made a habit recently of trying to reach users anywhere, even if that means going beyond its own Windows operating system, is bringing the add-in program to versions of Office running on devices powered by Apple and Google. It’s also part of the company’s effort under Nadella to work more collaboratively with other companies, even rivals.

Rob Lefferts, a director of program management, last week demonstrated a tool from business software maker SAP that pulled data into Excel for iPad. A separate add-in from on-demand ride company Uber showed how Outlook’s calendar could send reminders to request a ride in time to make an appointment.

It’s not all ready yet. Support for Word and PowerPoint on the iPad is coming soon, Microsoft says, with add-ins for Google’s Android to follow.

“That was sort of the first step; here’s our road map,” Lefferts said in an interview. “We’re going to make these run everywhere that Office runs.”

Lefferts said about 3 million people a month use Office add-ins built by other companies. Not a bad start, Lefferts said, but there’s plenty of room to grow. Microsoft says there are about 1.2 billion users of Office worldwide.

Add-ins can be tailored by individual developers and companies to cover a range of uses, from corporate lawyers writing contracts in Word by bringing in preset chunks of text to marketers inserting live polls in PowerPoint presentations or students researching in an encyclopedia.

Microsoft isn’t demanding a fee for companies to tack their programs onto Office, Lefferts said. Rather, the goal is to make Office a better program by tailoring it to specific customers’ needs.

“The business model is quite simple: If people use Office more, they’ll love it,” Lefferts said. “I am explicitly out there just trying to drive usage of Office, and when I talk to partners and third parties it’s a great conversation because the way in which I succeed is by making them successful. Getting them users, too.”