You could try setting the variable type to 'adapt to type' This may work, but note, if your dll trys to modify the string array, you may run into problems with the labview memory manager... Basically (I think I'm correct on this...) labview stores strings in a format where consisting of the start address and number of bytes to follow... If something alters this string out of the control of labview, lv's memory manager gets all stroppy and usually kills labview...
May not be a problem in your case, but if labview suddenly starts crashing when you run a function from this dll, that may be the reason