Is this possible?

How can I save a sound and make it available in the sound library after I have created one on Halion Sonic 2? Example, I tweaked the hell out of the Mono Piano which comes as standard with Halion Sonic 2, I added extra layers etc. Now how do I save this so that I can use on other projects?

Try right clicking over the slot that has your instrument loaded.
Do you get the ‘save as’ option?