My suggestions:
1) Obviously, once you decompress the DAT, SuperLemmini can already handle the resulting LVLs, right? So, that part's simple enough.
2) Default parameters, or maybe the CustLemm / NeoLemmix ones as appropriate scaled to suit SuperLemmini's resolution. Although you may want to allow selecting options here - eg; fall distance is 3 pixels higher in CustLemm than others (including Neo).
3) Beyond that, I think the only other thing that needs to be done based on user input is letting the user enter a name for the pack?
4) If any DATs do show up that use NeoLemmix-style oddtabling, they'll almost certianly follow the CustLemm level ordering, so that means for example, if it oddtables Rank 3 level 1, that's the 5th level in the pack.
One other thing to be aware of is NeoCustLemmix packs with secret levels. Obviously SuperLemmini doesn't support secret level triggers, but you could still make the levels themself playable. In this case, keep in mind that the 10 normal levels (2 per rank) come first, then followed by the 5 secret levels (which may not exist in the DAT if the pack in question doesn't actually use any secret levels). This is important for oddtabling too.
Just to keep it clear, the correct ordering is (keeping in mind the last 5 only apply to NeoCustLemmix packs):
1/1
1/2
2/1
2/2
3/1
3/2
4/1
4/2
5/1
5/2
1/3
2/3
3/3
4/3
5/3
You may not want to worry about dividing up the custom pack into ranks (as they're probably not made with any special attention to ranks anyway), but this is also important for the oddtabling.