Sheet Bend Rope Extension
Notes and Resources:
This method of rope extension allows you to join a new rope anywhere along an old rope; it does not depend on end knots to stay put. It lays flat against the body, and can be easily adjusted after tying by sliding a bit of slack through the knot in order to fine-tune its positioning (for example, so that it doesn't wind up right on a collarbone).
It takes a little practice to get the hang of this method, but once you learn the motion it can be done very fast. I use this almost 100% of the time for extending ropes.
Note that it's very hard to tie this if the rope you're extending is slack; try to situate yourself so that you can keep tension on the old line while adding the new one. Sometimes I move my body away from my bottom while tying, to accomplish this, if I'm extending a rope that is still more than a couple inches long.
A bend is the name for a knot that joins two ropes, and a Sheet Bend is one of the most popular. This slight variation doesn't have a standard name, but could be called a Sheet Bend on a Bight.
Can this be used for suspension?
For tying suspension harnesses, yes, this knot is as or more secure than other common techniques. Compared to a collapsed lark's head, it is better-studied as a bend and likely more reliable. Compared to a non-collapsed lark's head (i.e. one just holding against end knots), it is both more reliable (because end knots can come out), and stronger (because it loads both sides of the doubled rope, as opposed to if ends are not perfectly even, a lark's head only fully loads one rope).
I do not recommend extending suspension support lines, regardless of the bend used, because it can cause it to be difficult to lower in emergency situations (due to the knot getting stuck somewhere in the system). Use long support lines that you won't need to extend, and don't use the leftover rope from harnesses as an upline. But, in some weird edge case where you did want to extend a support line, this might be a reasonable technique to use (except in exotic synthetics, where it may not be secure). I would not use this for rigging a hardpoint; more secure techniques are better suited for above the ring.