I would think spells are the easiest things to add to LBB D&D, provided you keep the powerlevel the same, as in restricting them.
Agreed, spells are easy to add to OD&D.
Actually my biggest complaint with LBB so far is how clumsy the charts for combat are - and thats more an editing issue then anything, though I think Im going to just simplify them down to Thac0 progressions instead.
You might take a look at the options in the Swords and Wizardry retro-clone. The formated edition isn't ready yet, but a doc file of the rules can be downloaded from the
Swords and Wizardry web site. One of the options is the now fairly standard Ascending Armor Class and D20 roll system. This makes purists cry, but things like this were done in the 1970s. Heck, for a while I used combat table where AC went up and rolls were made on d100 and no one then said that wasn't real D&D.