Don't know if this has already been mentioned... Sometimes you just want to add one or two sounds to an existing drumkit. Since classic lift & drop does not work here, I discovered a workaround for this:
Lift the drumkit and drop it on tape. CAUTION: the beginning contains a lot of noise, it's the meta data of the drumkit! Don't modify that part! Go to the end of the tape portion, and record the sounds you want to add to the drumkit (you may also cut some existing sounds you don't need to get some extra sample time). Be careful not to exceed the maximum drumkit sample length of 12 seconds. When done, join the new recorded slices to the rest of the drumkit (SHIFT + CUT). Then lift and drop back into DRUM mode. Now you just need to map the new sounds to the keys (adjust sample start and end points, etc.).