You are allowed to enter N America twice provided that one of the entries is a transit either to or from South America, so that isn't a problem.

Moreover, intercontinental segments (which is what both the SA-MEX and the MEX-Europe segments would be) don't count against any total except the 20 segment maximum. That is fixed - you can't do 21 segments by buying additional segments. Moreover additional segments can only be within continents, not between them, so there is no way you could buy SA-MEX as an additional segment on the xONE4 ticket.

Having said that I don't see why you couldn't do the SA-NA (wherever to MEX) segment as a separate ticket if you have already hit the 20 segment limit, but in that case you need to be looking for which of the 21 segments you want to do would be the cheapest to replace with a standalone ticket.

In summary, if you aren't hitting the 20 segment limit then you can do what you propose without the need to buy additional segments. If you are at that limit then you need to replace one of the other segments with a standalone ticket.