There is no black magic in charging a 1-cell battery, just supply it the right voltage at a reasonable current limit (the "C" factor you may have heard of - 1/10 C meaning one tenth the mA of the battery's mAh rating being a sane default), it can be done with the generic 4-knob "bench power supplies"
Bonus points if you provide a thermal cutoff in case you got something wrong (or the battery fails at just the wrong time), but it's not a required thing (and I'm not sure if the DSi implements it anyway, the SP/DS/Lite don't)