

Add these before the name in the Smodel attribute in the Character.xml to get a superform:
k = Kaio-ken Sonic
s = Super Sonic
s2 = Super Sonic 2
s3 = Super Sonic 3
m = Majin Sonic
l = Legendary/Beserker Super Sonic
g = Super Sonic God
b = Super Sonic Blue
bk = Super Sonic Blue Kaioken x10/x20
r = Super Sonic Ros 
ui = Ultra Instinct Sonic

Example:
<char model="son.b3d" smodel="bson.b3d" bighold="0" metalsteps="0" skates="0" hoverruns="0" motorruns="0" norunsteps="0"/>