Li-ion is totally different than NiMH/NiCad in a lot of ways. For one, the cell voltage is at least 2x NiMH/NiCad/Alkaline/etc. It's not like there's two cells there in the pack to get it to 3.7v, it's still a single cell.....
If you abuse 'em, they'll blow up in a fairly unpleasant fashion. Prolly better to just start over and either homebrew, totally convert, or buy Li-specific gear.