Is it possible to add oil to the hydraulic disc brake reservoir to increase the stiffness of the lever, without bleeding the brakes?
There are 2 types of hydraulic brake systems, referred to as open systems and closed systems.
Open systems have a reservoir, with a cap, usually on the brake lever, above the master cylinder. These systems, when proper care is taken, can have fluid added, or hoses trimmed without rebleeding the brakes.
Closed system brakes, like most older Hayes brakes, (HFX-9, Nine, Original Hayes) can't add fluid, because the only access to the fluid reservoir is through the bleed ports.
Whether you can add fluid or not, if your brakes need to be bled due to water or air contamination, then simply adding fluid will not fix your problem. If you have a simple fluid leak, with no contamination, then it may work, although you should also look for the source of the leak, and fix it.
Bleeding a brake is not a difficult task, and usually requires few, if any special tools. If your brakes need to be bled, look up the manual, and give it a go. You can't really do any permanent damage, and if you follow the directions, you shouldn't have any trouble, anyhow.