My question would be - why? The Canvas Size dialog is set up to allow you to precisely scale a canvas without scaling the contents or to precisely add pixels to one or more sides (e.g., move the anchor to the left / right / top / bottom and add pixels on the opposite side).
A quick Google search through "keyboard shortcuts Photoshop" yielded many useful shortcuts, but it would appear that there's little to no keyboard control over this built into Photoshop.
You could create a custom shortcut using these instructions, but I'm not sure if you'd actually be able to create a shortcut for this since there's no initial way to control it using the keyboard (hitting "tab" in the dialog box just moves you between the W and H measurements).
prompt()
input dialogs to request the height and width, and then feed that to the resize canvas action after setting the anchor position.