I am "still buying the stock as of this morning," Ackman told CNBC's Scott Wapner in an exclusive phone interview Friday.
Ackman said he seeks "transformational change" at ADP and thinks its profit margin can "expand by more than 50 percent."
"I am not seeking control of the company," he added. "We think it's a great company that has underachieved its potential."
The hedge-fund manager is not proposing a spinoff of any company assets or for ADP to leverage its balance sheet; instead he plans to nominate a minority slate of directors of "four or fewer," Ackman said.
ADP released details on the fight brewing between the Pershing Square Capital Management manager and the payroll processor. ADP confirmed Pershing had amassed an 8 percent stake through Friday morning and that the manager wanted to oust the chief executive.
Word that Pershing Square was building a position in the company sent the shares up more than 10 percent late last month. Pershing previously owned ADP shares from 2009 to 2011.
ADP declined to comment on Ackman's newest comments.
— CNBC's Scott Wapner contributed to this story.