Isn't MainStage a Logic app? I would imagine Logic EXS24 sampler/editor would be the one to make an instrument which works natively inside the MainStage?
Step 5 (Programming) from the tutorial below would be the one to tackle if you already have readymade samples. If it's just one long sample with all the notes in there, you'd have to split it to individual samples first. Then mapping the samples to the keys and saving your instrument in a format MainStage can handle.
5 Steps to Sampling an Instrument
https://omegastudios.com/5-steps-sample-instrument/