I want a tympani sound with a gentle strike and long decay

Part of the problem here is that, as far as I know, MuseScore only allows you to select from one SoundFont at a time.

I'm quite sure that there is an open-source tool out there which will allow me to create a custom sound-font by mixing-and-matching sounds from other ".sf2" files ... but I don't know what that tool would be, nor how to use it. (Pointers would be extremely helpful, that's what I'm saying, because "I am new to these woods, 'tho not to computers, and I do not know my way around. I do not wish to be eaten by a tiger.")

I have already seen how any instrument can be attached to any of the sounds in one ".sf2" file, and how, through "staff text," different channels (of those sounds which have multiple channels...) can be selected.

It seems to me that custom-assembly of a soundfont which does include the additional sounds I need (ideally with a tool that would let me add a soundfont as a channel, would be "the cat's meow."

Uh huh, I could schleb around the Internet for a long time looking for this... so, can anyone who has already "been there, done that," please tap me on the shoulder and say, "go that-a-way?" :-} (P.S. I'm using a Macintosh... and I'm cheap, too. I'd prefer open-source.) So far, the only tool that I see anywhere is "PolyPhontics," which isn't free, and the limited videos don't say anything as to whether you can import from a ".sf2" file, nor anything about channels. (Oddly enough, you can't view their manual on-line without downloading and installing the product first.)