Following PF rules, as I understand them, you look at the CR of the imp. That is the ECL of an imp with no class levels. 2, I believe.

So, at ECL 6, you'd be an imp 2/Sorc 4.

Now, whether you lose your racial hit dice or keep them, I don't know. My group has houseruled a lot of monster characters and we tend to drop them, but I don't know if this is the actual rule as I don't have the book with me currently.