Your tricking your nervous system to over stymulate. There are neural benefits because you can consciously access that higher level of neural response after regular use of this type of overloading.
I would say that either adding the weighted stuff for 25 to 50% of a regular work out only maybe 2 days a week max with resting days in between would be the way to do it without risking injury. If you try to do it everyday/everywork out for the whole thing, your going to risk repetitive stress injuries and fatigue (which increases the degrade of form and technique) induced mistakes/injuries.
A simple and economical way to do this is to simply get a backpack (properly worn) and load it with your stuff (books, keys....) and carry that around during the regular day. Even doing some low to no impact calesthenics (push ups, lunges, squats, pull ups, dips.....) shifting the pack from in front to on the back as needed would be fine (with reasonable load). The wt. is kept close to the core of the body, so it is less likely to cause injury in the nature of a 'tennis elbow' way that ankle/wrist wts can do to users.
As mentioned before, lower back compression from the load can cause problems. make sure that you are recovering AND doing excercises that will improve that abdominal/lower back strength to support the extra wt.
Don't over do it! Even if it 'feels fine' or 'doesn't feel like it is working' stay to the rest/work cycle. With the lower back, if your always 'feeling' it then your working it too hard and you will 'always feel it' because of the injury.
