Combining BCS top-extending with BCS replacement (for music)

I'm following Talon's tutorial on adding new music, and what he says is that Weidu can add tunes but chooses tune ids dynamically, like it does most things, so that proper tunes are played no matter what the installation. Thus, if I want to change the daytime music for the Nashkel Fair, I need to put this bit of code in the tp2 file:

MKDIR ~music/FDAY~
COPY ~Fair/FDAY/FDAY1_#.acm~ ~music/FDAY/FDAY1_#.mus~
ADD_MUSIC ~FDAY1_#~ ~Fair/FDAY/FDAY1_#.mus~
COPY ~Fair/AR4900.bcs~ ~override~
REPLACE TEXTUALLY MUSIC ~%FDAY1_#%~ 

My question is, can I use the REPLACE TEXTUALLY command with an EXTEND_TOP instead? I don't want to override the area script for the Fair entirely with my own, I want to add my bit to it, with a SetMusic action, and then do the text substitution.