Need Keyboard-Only Method for Drag-Copying Formulas

I've created a report spreadsheet that uses formulas. I'm also writing a process document detailing how to use the spreadsheet.

If a user needs to extend the number of rows that contain a formula, they can click in the last cell that contains the formula, then drag the cell border down to add the formula to however many rows they need, automatically updating cell references.

Pretty standard stuff, but what I need is the way for keyboard-only users to do this.