It's not the BAM files themselves; the walking speed, selection circle size, etc. have always been notorious for being hardcoded in the originals. According to Infinity Animations readme, you can only change the selection circle size for new animations, but the walking speed adjustment has never been implemented.
2. Which part of of the bam files of animations decide the walking speed? And is it possible to change that of animations already exists?
Still don't know how to change that in my BGT game...
It wasn't until BGEE and the now-obsolete EXTSPEED.2DA (externalized walking speed) that modders could finally change the walking speed of each animation.
Tough luck, I'm afraid. The only options for BGT seem to be changing the walking speed through CRE files or an appropriate opcode; however, there may and will be issues depending on what you want to achieve.
Yes, that's what I choose by now. but I think new animations in contents in IA have various speed, but I don't find too much differences, only the count of frame, which confuse me.