Can the VNAV Path angle be modified in a Boeing 737 NG?
In the 737 NG, the descent angle is never entered by the pilot into the FMC. One enters a Cost Index, or alternatively a descent speed that's desired, and the FMC, considering forecast winds, computes the Top of Descent point and the angles that it expects to fly in order to achieve that speed.

But, it never tells the pilot what descent angle it's planning to use (although you can see the present vertical angle to a specified waypoint/altitude). Whatever the angle it's planning, you can steepen it or shallow it by increasing or decreasing (respectively) the planned descent speed or Cost Index.

But, unlike some other FMC's, the 737's never explicitly tells you the planned descent angle for enroute operations. 

(On an approach, it's a different story; there your gradient path is coded with the waypoints, and it is displayed on the LEGS page. But the user can't change values in the database like that. Speeds, yes; GP angles, no. A pure visual approach, with only a runway-end waypoint is the ONLY time you can specify whatever angle you wish... but that's only to the runway itself, and not at all usable on any other leg.)