If you haven't used the PCB Footprint wizard (which will place pads on the exact pitch you specify), you can add pads using the Type Coordinate or Add Multiple pads items from the right mouse context menu when actually adding pads in the Footprint editor.

You can also place pads manually onto the grid points. In the Grids dialog on the Settings menu. you can specify any appropriate grid with different X and Y axis intervals where the pitch differs between rows.

You can also use the Add Multiple and Replicate commands to produce repeated patterns of pads (as well as other items).

Remember that by selecting a whole row or column of pads, right clicking and opening Properties, it's possible to edit the common co-ordinate to move the whole line sideways.