I have a question. Is there any way to make a kensai /assaasin, whethwer through SK, NI or other? thanks in advance.

No, there isn't.

The game only allows for ONE kit to be applied to any character.

The only suggestion I make is a RECENT edition of SK would allow you to cut and paste all of the "effects" of a Kensai - the hit, damamge bonuses - over to an Assassin character. This would effectively be a Kensai/Assassin, but would only list as a Fighter/Assassin. To get a Figher/Assassin, you'd also need SK to apply the Assassin kit, and possibly to paste over the abilities too.

To GET the proper abilities, create a new Assassin, level up to where all the abilities are present, then copy across BEFORE you go into the game. Otherwise you might get the wrong ones.

Do the same for the Kensai abilities.

DON'T try to do it the other way around, because you CAN'T get the thieving abilities across without some seriously advanced knowledge of the code.

Good luck

How about a better idea, don't try it at all :). Oh, and I doubt you would have very good theiving abilites with a multiclassed Assassin.