A Lipo is just a battery. The cell voltages are rated higher than Nimh. Nimh is 1.2 v per cell and most stick packs are 6 cells so 7.2v total. Lipo cells are 3.7v each so a 1S is 3.7 and a 2S is 7.4v etc.
A 2S Lipo stick (or saddle for that matter) will work fine on a system designed for nimh sticks. The biggest watch-out will be your ESC will not have a low voltage cut off at 3.0v (per cell) so either run much shorter that you can or get a Lipo shield or similar like
these.