How do i implode all voices of a midi to one voice?

The entire point of a piano concerto is that the virtuoso piano part is accompanied by an orchestra. If you combined all the parts, it would be unplayable - and frankly, an unlistenable mess.  They ARE instances where orchestral works are made into a REDUCTION, more often for two pianos, or for one piano, 2 players ("4-hands").  There are also - primarily for rehearsal with singers - reductions of opera orchestral parts, mashed into a piano part - with the essence retained, but much detail sacrificed. They are utilitarian, and usually very tiring for the pianist to play.  So your idea has little application.  I own software that easily would do it - but you will end up with a mess.

Obviously it depends on which application you're using. On Sibelius you can simply select the lines you want to reduce and click the "reduce" button (if you don't know where that is just type "reduce" into the search bar in the ribbon) and then select which instrumental line you want it reduced into.

As Mamianka says, it won't be playable. It won't even be readable - it will be a gigantic mess with even 5 or so instruments per line, let alone simply shoving an entire orchestra into two lines.