There isn't a way to get a certain riff to happen at a certain bar within a style.

You can set a riff to play on a pre-fill bar (2 bars before a part change), or on a fill bar (1 bar before a part change). You do that by setting a riff's options - bar mask to pref-fill or fill, and setting the weight to 9 (always happen).

If that isn't what you need, and you need a specific bar # instead, then
John's suggestion is a good one (a custom style, with a style change).

Have Fun!
Peter Gannon
PG Music Inc.