This extension converts selected frames that contain frame-by-frame animation, classic motion tweens, or shape tweens to keyframed intervals, just like Convert to 2s, except that you can set up any frame interval (3s, 4s, etc).
Note: When I tested this in Flash CS5 (Mac) it ran perfectly. In CS3, not so much. So use with caution. Same goes for Convert to 2s. I’m not sure where the discrepancy comes from, looks like a Flash bug, but I’ll fix it if I can.
Download
(Compatibility: MX 2004 & up)