This is imminently doable, and surprisingly simple.
Zelda 1 stores its text in Bank 1, $[4010-800F]. The pointers to the various text messages are at the very beginning of that bank, comprising the first several lines. All you really need to do is alter them accordingly based on the starting points of your new text strings. Do you know the Zelda 1 text format, and how to convert ROM addresses to their CPU equivalents?
Bank 1 has lots of unused space, especially towards the end. You shouldn't have to expand the ROM or do any weird bankswitching.