To convert .bls to .blrs, you'll need 'Node.js'.

Just put the save file in this directory, run 'node start.js', and all .bls saves in this directory will be converted to .blrs.
After that, simply put the blrs file in 'Add-Ons/Client_MainMenuBuildShowcase/builds/' and it will be added to the pool of random builds to load on startup.

NOTE: Don't put extremely large builds in here. The game can handle rendering it, but loading the build initially will slow the game down dramatically.
NOTE2: Builds with custom bricks require that you copy the missing brick datablocks into 'dataBlocks.txt' (make sure 'brickFile' is an absolute filepath).