Update to the newest version of MultiMan and install BDEMU.PKG (attached to the MM thread, use the paperclip icon on the thread title). Make sure you are putting the game in a location that MM is set to check like X:\Games on your external drive. Once MM sees the split game, copy it to the internal HD and MM will unsplit it for you.
the LA-Noire i downloaded was already split for >4gb files. I copied the already split files, from my ntfs external, to the internal hard disk of the ps3. But it did not work. Then i copied the files to the fat32 external HDD and then copied the backup using MM to the internal HDD of the ps3 and it was able to copy. I don't know if there is some bug in the MM or in that particular release.