No, you definatly CANNOT connect the neutral and ground together on the load side of a circuit. "This is called a false ground or a bootleg ground. The neutral conductor can only be bonded to the ground conductor at the main neutral bus, where a large copper conductor carries all the return and faulted current back to the earth. If the neutral and ground are connected anywhere else in the building, all grounded metal becomes part of the neutral conductor, constantly energized and creating various voltage potentials." This is really really dangerous.
http://www.stevenspublishing.com/Ste...a?OpenDocument
And, you're correct, you can't just replace 3 prong receptacles in place of 2 prong unless you have a ground available in the box. What type of 2 wire cable do you have? Is it armored cable? If it is you "may" have a ground but you'll need a voltage detector to know for sure. The following link will tell you how to test for one and the circumstances required in order to install a 3 prong receptacle. Otherwise you can install a GFCI in the first receptacles box on the circuit and connect the outgoing wires to the LOAD side of the GFCI. Then all receptacles downstream can be replaced with 3 prong receptacles but must have the sticker installed on them saying No Equipment Ground.
http://www.codecheck.com/250_50_commentary.html