2 common methods:

1) insert a velocity envelope (right click the event, insert velocity envelope). Click on the envelope at the frame you want to freeze and set the point to 0. You'll probably need to add a point immediately before that frame at 100% (normal speed). Now you can drag that clip out as long as you like, then click to add another point at 100% to get it back up to regular speed.

2) split the event immediately before and after the frame you wish to freeze, then add a velocity envelope on the new event you created (the one with your freeze frame), and set the envelope to 0. Drag that event out as long as you want.

Greg