Blake, 
    You can put your drip tip on your cartomizer (just remove the plug and replace with the drip tip  ) and then when your carto is running low, drip down the inside edge of the drip tip to top off the carto. 
 
Ooops, was on page 1 thought I was on page 2 and missed the above posts.   
 
Don't drip directly through the drip tip into the carto; drip so the juice will run down the inside of the drip tip.  Or as Day points out, just pull the drip tip off and top off.