Hi, i have the player animations in .XSI format and i have the plugin to import .xsi files into max, but, i want to know if there is a way to modify the animations using character studio, or save the current ones as .bip files
No, it isn't possible right now. When the .xsi is imported the bones end up as helpers, they are still animated correctly, but it doesn't help you when you need the animation applied to bones.
You'd need to use SoftImage xsi and export as BHV I beleive. Which Max allows you to import onto bipeds. Another method is to use an app called I beleive Motion Capture Studio. It uses the FBX file format. It allows to import xsi and export as a couple different formats. Hope that helps.