No, it cannot be done.
Points.com allows you to trade certain miles for others, but the losses in the transaction are not worth it.
As for how people can afford international business/first ...
1) UA is partners with Chase UR, so getting another Chase UR card will let you transfer those points to UA. UA also allows one ways.
2) Lifemiles has a good cash+points scheme, so you don't need that many points. They also allow one-ways.
3) US Airways sells miles for pretty cheap (like right now, $1900 for 100K miles). They don't have the Grand Slam this year, but the past few years it's been easy to accumulate >100K without credit card sign ups.