I don't know about an external for that, but I can tell you that unless you already have a highend turntable, LP's in mint condition, and a whole lot of time, you may be disappointed in the quality. The gear to do this may cost more than replacing the music.
We had 50 or so LP's and a box of old favorite cassettes. Also an old, but at the time of purchase high end turntable, and I hooked it up to our pc last winter. Even with current software and a hours work per LP I still had scratchy sound. Nothing I wanted to actually listen to. In the end we decided to replace what we could with CD or MP3 versions and abandon the LPs to Goodwill.