From your diagram, it doesn't seem you know electronics/electric stuff too well, so I would not suggest you try to do this yourself - most likely you will cause damage to your amp.

If you are set on using that cabinet - and don't want to get a 2nd matching cabinet - it will need to be modified to having two input jacks, each at 8 ohms.  I would guess that the cabinet has four 8 ohm speakers, each pair wired in parallel (to give a 4 ohm load), then the two pairs wired in series to give an 8 ohm load.  You woudl need to add a 4 ohm Power Resitor (in series) to each pair, then each pair would be connected to an input jack.  Trying to run both left and right amp ouputs into 1 mono input is going to result in phase issues when you listen to it.