Technically you can convert stereo to mono with any audio software, but as Bobber says, the original mixes were different. Moreover, when you combine both channels, the relative volume is changed. For example, if you have a song with the vocal on the centre (both channels) and the guitar only on the right channel, the direct mono version will have the vocals at the same volume but the guitar at half the volume. That's a problem with many Beatles recordings mixed as extreme stereo, because if you convert them to mono you will have some sounds buried and will not be fully appreciated.