Edit existing event: Terra's Flashback

For Terra's Flashback, you first need to create your NPCs and show them. One way to add code is using the B2 command to call event code from elsewhere in the ROM. Let's say you place your new code at $F31122, then the B2 command parameters would be that offset calculated from $CA0000. $F3 - $CA = $29, so you could place a B2 22 11 29 at $CCA44F. At $F31122 you would place your two create object command you just overwrote (Create objects $1D and $1A) and after put the create object commands for your other NPCs and show object commands ($41) for those last two NPCs. Then end your new code with $FE. The show object commands for objects $1D and $1A are a bit later in code but that doesn't matter.

To insert an action when dialogue pops up, use the same B2 command at $CCA466. Put the overwritten command $5C, insert your GFX action for your NPC, then the dialogue one ($4B) and end new code with $FE.

When you use the B2 command, you can't split a command, meaning if you need to remove 5 or 6 bytes to fit your $B2 command ($B2 is always 4 bytes), do it and after the $B2 command put one or more $FD (dummy command) depending. $FD will act as a "nothing" in this case.

This is how I add event code, the rest is just following examples you have in the event dump. There's also great event resources on the wiki.