I'm really no expert in this (I just taught myself how to use the decode tool yesterday with those helpful tutorials), but I just put it after the last old building_levels_table definitions.
Also an odd thing - column unknown6 (type: string) in the new table is empty for all but one entry (new level 5 DLC building), but PFM tells me that column can't be empty (I assume because it's defined as a string). Maybe I messed something up here, but the decode tool tells me that it should be fine. It simply doesn't work if I make it an optstring.