Edit the shortcuts.txt file.
Select the layer, then use the menu command: Layer > Add Marker. (or do what Mylenium said)
Here's a radical notion -- get a different keyboard. One with a numeric keypad. The problem goes away. You can get a replacement keyboard for 20 bucks or less.
There is a little icon to the far right of the Work Area that looks like a Marker. You can click and drag it to where you want the marker.
Also, CONTROL + OPT + 8 (on the top row of numbers) will open the Set Marker window and set a marker for you where your Time Marker is located.