While it's not the Dopesheet, but the Graph Editor, yes, there's a workaround to retime it. Animate the start frame of the build modifier.
So far the build modifier cannot easily be animated by percentage or face count (without using python). And the speed curve does not work with modifiers as of yet.
A workaround to achieve variable speed is as follows:
The build modifier is controlled by two values. Start and length. For this approach, length should be set to either 100.0 for easy calculation or to the facecount, if desired.
The trick is to animate the start value in respect to the current frame. Here are the rules: The modifier is stopping it's progress if you animate the start frame in parallel to the frame number. Start frame 50 in frame 50, start frame 90 in frame 90 (don't forget to set the IPO curve (sorry, I mean F-Curve) to linear or you won't be synchronous due to the ease in and ease out nature of the beziers).
The build modifier progresses forward if the start value rises slower than the frame number:
start frame 50 at frame 50
start frame 100 at frame 150 (offset of 50)
will make the build modifier progress at half speed until the 50% mark and then progress at full speed, if nothing else is animated.
Whenever the start value is identical to the frame number, the modifier is at 0.0 per cent.
So the formula with which to calculate animation values is:
start value = frame number - (length / 100 * percentage)
You find the values in properties>>modifiers>>build modifier panel
Look at it a little bit like two cars, one moving at constant speed (frame number) and another catching up or falling behind (start value of the build modifier). The distance between the two cars defines the amount of built faces.