Play them in! Do you mean in score view or in piano roll? Score view has a tuplet tool.
In piano roll:
- play or draw your run in on quavers/semiquavers (so the timing and length of notes is consistent)
- make sure the starting note of the run is in the right place
- Go to functions-time handles. Lasso-select your run, and bring the right edge to the point you want the run to end