Quote from Pedro Rocha »Is it possible to add new spell on the stack after just some of them resolved?
Yes. It can happen when the right conditions are met, such as what was discussed above.
It's not usually a "thing" in modern Magic anymore but it still does happen. Most spells or abilities are stacked in a sensible way and resolved to completion without further interaction. However such a thing was very common in old Magic, sometimes to the point of bending the rules.
A modern example might include Slimefoot, the Stowaway and Epicure of Blood and say... ten dead Saprolings. Slimefoot life loss/gain effect goes on the stack ten times then begins to resolve. Each time I gain life, Epicure's effect goes on the stack once, resolves. Then the next Slimefoot resolves putting Epcure on the stack. So on and so forth....
In this case, Misty’s ability resolved (and the land is now on the field) but, after this resolution and before the next spell resolves, may I add more spells to the stack?
Yes. Whenever the topmost spell resolves, there is always a "round" of priority where each player receives priority to do what they can. Only when all players pass priority does the topmost spell or ability resolve. Then the entire process is repeated on the next spell (which is now the top most spell or ability) to resolve from the stack. Most of the time, players will simply short cut the entire process and just pass on everything on the stack (such as if the player is tapped out or have no cards in hand that can be cast at that time).
I thought that after the two players pass in succession the stack should obligatory resolve everything from top to bottom in the exact order.
No. All players must pass in succession for each item on the stack from last on placed on the stack to first placed on the stack (or top to bottom if you wish).