I am not sure about trying to modify the routine - wouldnt that just allow them to log-in. it would do nothing about maintaining the personal user profile (address,email,etc)
I think some database conversion would be in order. I guess you could use some really expensive sophisticated software to do this - but - i think it could probably be done by hand - i have to do this with my existing store.
You might be best to ask the cs-cart support people when you buy the software. They will provide you a quote for a custom mod.