Sorry. LMMS cant do that with samples. What you ask for is called sample-stretching, and it is not as simple as you may think. You need to use something like audacity, and manually stretch each sample to correct pitch, but then you also need a Sampler, and LMMS do not have a native sampler yet, so you need to use a VST-sampler like Grace, and manually insert each of your samples on correct keys.
Its not a fast and sleek process :p
Try MDA-piano
http://mda.smartelectronix.com/synths.htm
It is very small in size, and you can just copy its dll to your VST-folder. Further its a quite good piano-instrument, so it is a win win