you need to find out if his forward motion is happening in the body part or the hip part.  (hopefully the hip.)  if it is the hip, then you can open the hip x, y, z graphs, drag the cursor over the entire second animation part, and drag that entire part over in x, y, and z watching the window so that you match the end of the first part.  actually if the motion is in body the technique is the same i guess.  what you are doing is adding (or subtracting) from all of the frames you need to affect at the same time.  you can do this for rotation too if the rotation is off.

 

also if you find that the body parts jump or do something really weird when you add two pieces, check that part to be linear.  two abutting curves in the graph can make for some wierd motion down the line.  very best bet, just key every frame before adding the second motion.