Depends on what level of work you want to do. Often people might buy a driver that regulates current (and maybe boosts or reduces voltage). Commonly LEDs will tend to use around 3 or a little more volts. Thus a common mod is based on the "amc 7135", check those mods out and see if that's the level of work you're looking for.
But the problem (as was stated above) is that alkilines (even D cells) can't put out very much power and if you do make them do it their run time will be SHORT. 3 alkalines discharging at 3 amps would get you around that 800 lm (real) mark when new but the voltage sags down under such high load very fast, like 5 to 10 min and you're below the 1.1v (or more) needed per cell. The 5,6,7th graphs are for D cells and show their run times at different discharge rates.
http://www.candlepowerforums.com/vb/showthread.php?64660-Alkaline-Battery-Shoot-Out At 1 amp you might get 2 hours runtime and around 200-250 lm (real lumens, maybe 300-350 emitter lumens).
The drop in route could have similar run characteristics as getting components yourself and putting it all together and I don't know what a drop in costs or too much about which one you'd want. Buying and assembeling components could cost something like 20 for the heatsink, 5 for the driver, and maybe 5-10 LED. For a total of not less than 35 and quit likely more you'd have a light that eats alkilines and isn't terribly bright. The plus side would be that you could also run 3x NiMH in this light if you wanted but at that point you might as well decide to make a NiMH light (I'm sure you can look up others who have done that here. hint: kevin's signature). AND if you were able to get 2 Lithium ion cells in parallel in it they would work too; and assuming they were decent sized, give good run times and light. But with the lithiums you're into another 35+ for batteries and charger and a little more for rigging them in parallel and you have basically made a new powerful flashlight instead of making a back up. So do you want to undertake a project that has a totally different goal from the one you were thinking of just because you have a host body for a flashlight?
TL;RD: It isn't worth spending money modifying a flashlight to run alkalines.
What is your current flashlight and batteries that you mentioned?
