Yes, you can use your MB char in Warband and vice versa. In character screen (C button), in the lower left corner, there is a Statistics button. There you should find the export and import functions.
Export your MB character and find the txt file in your MB\CharExport folder. Now the location of the next step might differ from OS to OS. In win7 copy that txt file into C:\Users\....\Documents\Mount&Blade Warband\Characters.
Next time you start Warband you can import the character. Remember, in order to import the character, they must have the same name. As you'll surely find out, you can edit the **** out the txt. Change skills, proficiencies etc. But the character doesn't carry over equipment, renown and honour.
Have fun
Or you can just export any Warband character. edit to your liking and import back to Warband.