All ALICE packs have webbing strips on their back and/or sides to which ALICE gear was intended to be clipped.  You can do the same with most MOLLE pouches, especially the smaller ones, whose MOLLE attachment loops are about as far apart as ALICE clips are.

I would have to check to be sure, but IIRC the MOLLE sustainment pouch attachment straps are much further apart than the typical ALICE clips, so they will not properly mate with the existing ALICE attachment points on sides of the LG ALICE.

The Med ALICE has a long, wide ALICE attachment band going around much of the pack, and attaching wide MOLLE pouches there is possible, but doing so would cover the existing pockets on the pack, interfere with the cinch-down straps and their adjustment, plus the MOLLE sustainment pouches are so long that they would not be well-secured by their straps, and would flop around quite a bit.  I suppose you could use a horizontal compression strap to tie-em down, but that adds to the issues.

Maybe there is a clever way to do this, but I think you are mostly confined to smaller MOLLE pouches which will fit onto the existing ALICE attachment points, and which, by themselves, are not so big as to cause problems.

IIRC, the so-called "100 Rd Pouch" and the canteen/utility pouch might be good candidates. Double-check pictures of their attachment straps and make sure they are about the same width apart as ALICE clips are.  Just remember that even if the MOLLE straps will fit the ALICE attachment points, the longer the MOLLE pouch, the more it will flop around, because the part extending below the ALICE attachment point is not properly secured, as it would be on a MOLLE mounting system.

I just ordered a down-east Alice frame.  I wonder if the locking buckles on the Molle sleep carrier will lock into the Molle slots in the frame.

The answer is yes.  I could get two of the toggles to lock into the frame.