This is far from a trivial task, since it requires remembering the state of every frame, or at least the difference between each frame, which SuperLemmini does not do. Still, I will look into adding it, but don't expect it to be in the next version.
Lemmix implemented this via the replay system. Let's say, for example, that a user is at frame #1000, and presses "back one frame". Internally, the game restarts from the beginning of the level, and (without rendering anything until it's done) executes the replay up to frame #999 then stops.
This does result in poor performance, so NeoLemmix improves on the above by using a combination of the "remember state" and "use the replay" system. Not every frame is remembered, but rather, the state is saved from time to time, and the replay method is used to cover the gap between the most recent save-state and the target frame. In the case of NeoLemmix, it saves every 10 seconds, and over time, discards some of the older states. The state at frame #0 is always kept; as is the state at every 1min interval. States at 30sec intervals are kept for about 3 minutes, while states at 10sec intervals are kept for about 1 minute. (All times here are in-game clock time, not wall clock time - no new states would get saved while the game is paused, for example. I might also be remembering the exact intervals / cutoffs wrong, but the general idea is right.)
Lix's system is similar, but with different timings, that were designed more with the multiplayer mode in mind. The basic idea is the same, though.
NXLV level support is planned, but isn't guaranteed to be in the next version. I'm not quite up to speed on the format right now, but I'm pretty sure it references objects and tiles by name rather than number, so I would need to think about how to work with that.
The best way would be to change SuperLemmini to also reference pieces by name. Such a system is a bit tricky to get backwards-compatibility with (though NeoLemmix has a way - will explain later in this post), but going forwards it makes things much tidier when pieces need to be added to or removed from a style.
However, if you're not willing to do that; NeoLemmix still supports older formats (for now - this is intended to be dropped in the near-ish future). These old formats, of course, reference pieces by index numbers still. In the "data/translation" folder of NeoLemmix, there's files that are usually called "translation tables", with an NXTT extension. These tell NeoLemmix how to translate the index-based scheme to the name-based one.
These tables are not designed to be used in reverse as-is (for example, modern NeoLemmix no longer has separate bodies and tops for exits, they're now one large object; so the translation table says "replace with this and move slightly" for the exit object, but "just delete this piece" for the exit top), but it could be done with slight modifications, or with some special logic on SuperLemmini's side. You're welcome to use the NXTT files, as-is or modified, if it helps.
You'll definitely want to read this:
https://www.lemmingsforums.net/index.php?topic=4334.0This explains the basics of how text-based NeoLemmix data files work. This is applicable to just about every text-based data file in NL, including both NXLVs and NXTTs. Pretty much the only exceptions are a few INI files used to save user configuration, which are just the basic "XXXX = YYYY" format you usually see in INIs.
This topic specifically explains the level format:
https://www.lemmingsforums.net/index.php?topic=4336.0There are a few tweaks to this format that are occuring with the next major update. The topic is already updated for this, and references to the old cases have been removed. Examine some existing level files or look at
NL's source code (Delphi) or
the editor's source code (C#) if you want to figure these out; I made an intentional choice not to keep them documented. Alternatively, you can simply tell people "NeoLemmix levels created for versions older than NL V12.7.0 need to first be run through V12.7.0's Cleanse Levels feature" (a feature that takes an existing level, and re-saves it in 100% up-to-date NL format; this can be used for just outdated NL levels, or for a few other formats including DOS, (Super)Lemmini and Lemmins).
If you're planning to make your own editor, you are more than welcome to use the NL editor's code as a starting point if it suits; it's under a CC-BY-NC licence (so is the engine's code). For the "BY" part, you'd need to credit both myself and Nepster, unless your starting point is commit 2a6218e or older (in which case only Nepster would need to be credited, as none of my code is in the editor prior to that).