Converting Minecraft .lvl to .dat

Is it possible to import/convert a Minecraft Classic server level to a Minecraft server level? I have a really old server, set up in McLawl r55, that I would like to open and explore in the current version of Minecraft.

If so, how can I go about doing this?

Yes, you can do this.

First, download McEdit. Open up your Classic .lvl and save it, in McEdit, as a schematic. Then, make a super-flat world and open it (or create a new one with McEdit), and paste the schematic where you need it to be. Save it as a Minecraft world.

(Remember that you may need to patch things up; I did this once and I had to seal things up because lava was coming in, and the floor could reach the void.)