Add hand positions to flat bar MTB I'm trying to make a more road bike from my old GT Avalanche (26 hardtail), `cause I've bought a new one for trails.

I had a 100 km ride last weekend and found that I really lack hand positions on the handlebars. What are my options? I don't want to buy a drop bar, since it's quite expensive and can cause problems with brake levers and shifters. Are there any good alternatives?
Bar ends are as cheap or expensive as you want and easy to fit. Being unfashionable, you may be able pick up a set for nothing from a local bike co-op or recycle center to try. 

Other add on's that you could try are bolt on aerobars. 

Butterfly bars used by tourers could be worth looking into, although these are not widely used outside that circle so can be hard to come by or expensive. 

Before spending too much money consider selling the bike and buying a road bike with drop bars for the same money. For no net cost, you will have a much better bike for the longer distances.