Are imperfect-information games in extensive form exactly the same as sequential incomplete-information games?
Two points:

An Incomplete-Information Game is not well-defined without specifying the beliefs of the players. This is specially relevant if you want to use the standard Bayesian equilibrium techniques. There is still significant leeway here - you may define subjective probabilities for each players, players may have inconsistent beliefs (look at Yildiz's excellent papers in a bargaining setup), etc. Recently, there have been some papers that deal with situations where a player is uncertain about the beliefs held by other players (see Gab Caroll's Robustness and Linear Contracts for a brilliant exposition)- but you may need to develop additional tools (like updating rules under ambiguity - see Faruk Gul's paper) to study these situations.

As for Harsanyi's transformation, the critical component of his 1967-68 paper is the common prior assumption (so every player has consistent beliefs regarding the uncertain elements of the game. Thus Harsanyi states that any incomplete information game with consistent beliefs can be remodelled as a game on imperfect information by introducing Nature's move at the beginning. There may have been extensions to the result in the past 30 years, but Im not aware of it.