Yes, but it would have been really tense to do so. A good summary of the document that describes the procedure in detail is here. That page also links to a lunar module operations manual that it summarizes. Here's the way it was supposed to go down:
On the assumption that, for some reason, the ascent stage engine and explosive bolts etc. couldn't be activated by the ascent stage, the astronauts had no choice but to do things on EVA.
Problem with that was they would have jettisoned their life support systems by that time. They had a backup oxygen supply that lasted for 30 minutes, so the entire thing would have to have been done within that time. Also, without the life support system, they would have had no cooling in their pressure suits.
There were then two procedures that had to be followed, with the first procedure being deemed safer and easier.
The first procedure would be to attempt to use the LM's descent stage batteries and a jumper to do what could be described as absurd: rush out the LM (remember, you only have thirty minutes and, if this procedure failed, you have to use the rover) and attach the jumper to one of the descent stage batteries. Leave the cabin unpressurized. Launch, and when you're done the burn collect the cables that are now dangling out of the LM and finally close the hatch.
The second procedure would be followed if the first failed. This involved going to find the rover (rapidly), and then using it as a jump starter just like the descent stage battery.