Green_Hilltop: You do remember it correctly, it's a class kit for Fighters in BG2 and was retroactively added into BG1 remake since it used BG 2's system. And BG2 remake has it as well, of course.
https://baldursgate.fandom.com/wiki/Kensai Yep, I have just been playing a new one (lost the old one when I accidentally knocked my external harddrive off the table, along with all my other saves, like a BG1 Paladin in the city, and I-don't-remember-what-else-off-top-of-my-head).
It may require one of the mods generally available, but there is the option to buy a version of Dak'kon's blade (a longsword+2 sold by Deidre in the Adventurer's Mart that grants an extra 1st, 2nd, 3rd, & 4th level spell) but it is not cheap —— costs 26600gp if Jaheira is negotiating with her rank of 15 in Charisma (so I hadn't generated enough cash to purchase it back during my old play-through) and I was just a Grand Master in longsword. (If one dual-classes from the start of
BG2 the character is close to achieving the prerequisite 8th level by the time the party exeunt from the first Irenicus dungeon.) Interestingly, this time, I have opted for a generalist fighter preparation, instead. (I'm not sure of the wisdom of this strategy, yet.)
But, back to the OP, it is quite reasonable to, say, take three or four ranks in Katana and a pip in Schimitar to be proficient with the other Asian swords, the Wakizashi (which a Katana proficiency will provide access to, anyway) and Ninja-To. (It would chiefly be for roleplaying, though, since the whole point of the Kensai is to specialize on one single-handed weapon, to the exclusion of all else. )
Once the character reaches the Kensai level-plus-one, the dual-class (Mage, if you're imitating Dak'kon, but conceivably any class you wish to play with a Samurai base) will have full access to these exotic weapons.
More realistically, I suppose, one would dual-class a Kensai with a cleric, but that is practically a Paladin without two-handed weapons.