You're going to need to open Peach and Pit's FitFighter.pac files in separate instances of BrawlBox (at least v0.71 is recommended). Once they're both opened, open up their ef_fighter sections. Minimize Pit in the meantime and put your focus on Peach.
Under the ef_peach, you're going to see an EFLS, REFF, REFT, and two BRRES sections. Since Peach's Heart is a REFF, GFX, you're going to need to open up the REFF section and look for the appropriate GFX. To make things easier for you, the GFX you're looking for is PtcPeachAttackHi3 (remember this entry name). Though since this is a REFF GFX, it's going to have sub entries. Again, to make things easier for you, PtcPeachAttackHi3 has two sub entries; PeachAttackHi3Tsubu and PeachAttackHi3Flash. Once you've found the entries, export them. Make sure they have the exact same name as the original entry.
In some cases, the character might have something extra in the REFT section for a certain GFX. This is true in Peach's case, so you're also going to need to open up the REFT section and look for the textures that are associated with her heart effect. To make things easier for you, the textures are named ptc_peach_heart1 and ptc_peach_heart2. Export the two textures to the same place you exported the REFF GFX. You can close Peach now and put your focus onto Pit.
When Pit is brought back up and the ef_pit section is opened up, you're going to need to open the EFLS, REFF, and REFT sections.
Since you're adding a GFX to Pit, you're going to need to add an entry to the EFLS section. This is done by right clicking on the EFLS and clicking on Add New Entry. Now scroll until you get to the end of the EFLS section and you'll see a <null> entry. Click on it. Remember when I noted to remember the main REFF entry name? You're going to rename the <null> to the main REFF entry you exported. Also, since the GFX doesn't use a model, set the UseBrres option to False.
We're almost done. Just two things left to do.
Open up the REFF section and add as many REFF entries as you exported from when you had Peach in your focus. In this case, you'll be adding three REFF entries. This is done just as you were to add an EFLS entry. When you've added the entries, you need to replace them with the exported REFF from Peach and rename them to match the exported names.
Once that's done, finish things off by importing the exported textures into the REFT section and save afterward. Don't close just yet, as you need to get the brand new GFX ID. This is done by counting down the EFLS entries from the top, starting at 0. To help you save a bit of time, if we assume you're adding Peach's heart GFX to an unmodified FitPit.pac, then the ID will be 20. Once you've gotten the ID, you can now close Pit out of BrawlBox and open your edited file in PSA.
Once in PSA, you're gonna need to insert an External GFX command (11010A00) into the Sub Action of your choice (usually the GFX tab). For the Graphic Parameter, you're going to need both the ef_pit ID and the EFLS ID (both in hex). In this case, your newly added heart GFX onto Pit would be 00180014 (18 is the hex ID for ef_pit and 14 is the hex ID for your newly added GFX, as 20 is 14 in hex). Make sure the Scale of the GFX is higher than 0. Save and test afterward.
