You might want to consider grouping an object onto itself. Doing this gives you a second pivot point which can be constrained to an object, but leaves your object's primary pivot point free for animating like you choose.

For animating a character picking something up, one method I've seen is constraining it to locators, rather than specific joints. You can then parent a locator to a joint if you want.

If you constrain an object to two locators (or objects) you'll see in the channels that you can animate how strong one constraint pulls in relation to the other. If both are at the 1 value, for example, your object will constrain between the two points. So, you can animate the constraint between these two points seamlessly.

Good luck!