Yes, you can pull roving off the drumcarder! Just use a diz, even an impromptu one with a hole punch and a yogurt container lid to pull roving off the drumcarder oncs it is full. The hole size will dictate the roving width (mostly -- also how much you start with).
You start at the right edge (standing at the front of the machine where you feed fiber in) and where you would usually break the batt, you lift about 1/4" of fiber up from that edge, feed it through the diz and start rotating the drum carder so you can continue to pull the fiber up, feed it through the diz as you go. The trick to continuing past the first rotation is to ensure you have enough fiber presenting itself on the right edge as you come around to the starting point each time.
One thing I found trying this technique on carders -- if the carding cloth is not laid on at an angle, it's a bit trickier getting the roving to be continuous around the drum. When it's \\\\\\ rows of teeth like on the supercarder, the next round usually presents itself at the end of the first round, starting at the right edge as you look at the front of the machine. When it's ||||| rows of teeth around the drum, you reach the end of round one at the start of round one -- so you have to try to diz up roving a bit more from the left side, assuming you are dizzing right-to-left as you look across the top of the drum, as you approach the starting point, to be able to get to the next round as you complete the first trip around.
(posted by me on spindlers this day)