Probably convert to 29.97 progressive and then drop onto a 23.976 timeline?
I've done it with 29.97 SD footage--you get about 25% slow-down by dropping such footage onto a 23.976 timeline. I'm assuming you can do the same with 50i. You might have to export and re-import though so that the footage is in a format that the Avid 23.976 timeline can understand.