The manual states that stems are made by Artiphon with no suggestion that users can make them. In theory it might be possible but having seen what it will involve doing I would not recommend it unless you can persuade Artiphon to provide a means (an app of some sort, probably) to do it.
I wonder whether you mean that you want to replace the lead preset in an existing stem song. My comment above still applies really, but.......
You would have to create a lead preset with the 8 samples. (I would want to check the format of the presets in stems - it might be different to ordinary presets.) The samples themselves would need to be in the proper format and in matching key and bpm. Then you would have to create a new stem preset file which would refer to the stem presets for the other modes (Drum, Bass, and Chords). I think they would also need to be stem presets, not ordinary presets. (I can't see it working as you want otherwise - but I might be wrong!) And then also include a reference to the Lead preset you created in that stem preset.
I considered the possibility of making stems myself - but I saw it would take a lot of careful work and the potential to brick your Orba in the process, so I dismissed the idea pretty quickly.
(If you were trying to use previous existing stem material there might be a legal issue about it - but Artiphon would have to answer to that issue.)
That would be a long explanation. I would suggest looking for an example of a sample based preset from my earlier ones or by others. And also read through my explanation to a query by Rusty Perez - it was about chords but it covers a lot of the essentials - here: https://artiphon.freshdesk.com/support/discussions/topics/44001021315/page/6
I don't know whether you can access samples made on the Orba for this purpose. It might work - I just don't know, haven't tried it.
Be warned: getting things wrong - even a tiny typo in the wrong place can cause big problems even bricking your Orba (as I know to my cost).
Using the scripts on my GitHub page allows you to create stems like the Kimbra preset
Marc Ponsioen
I would like to ad 8 different stems to Orba 2 lead. Like you find in the songlibrary in Kimbra : replay! Can i do that and if so how do i do that ?
Grtz Marc