I have done it by creating SF2 soundfonts, with a program called Polyphone. It's a great, free soundfont creator/editor with tons of capability...or you can something simple, of course.
In Studio One version 3, Presence could use soundfonts, and I'm guessing that it still would work in the current version.
Anyway, here's a link to Polyphone if you want to check it out: http://www.polyphone-soundfonts.com/en/

Can I import wav or mp3 file into Presence??

I don't think you can import wave or mp3 samples. At least not directly.
You can load "presets" with the following formats:

There is a paid version of Presence called Presence XT Editor:
http://shop.presonus.com/Presence-XT-Editor
But if you do something like that (detailed, flexible crafting of an instrument)- well, Polyphone would do the same.
* I did check on the soundfont thing- the current version of Presence will still play soundfonts.
But stay tuned... I may have a better idea...

I was thinking also about Impact XT. You can drag and drop .wav files onto pads. It also has multiple velocity layers. It's referred to as a drum sampler, but you can in reality use it for anything.
However, you would have to upgrade to at least Studio One Artist to get Impact XT.

Here's a video on Impact:
* Go to 4:00 to see how to import files, and a minute later it shows the supported formats, which does include mp3.