A BW hierarchy can be loaded & maintained via a flat file. The approach is to maintain proper Child & Parent nodes in the file and upload the file in BW syatem.
See below the steps:-
1) Create a BW hierarchy data source and keep the fields like Node ID, Infoobject, Node name, Link name etc. See the below structure.
2) Create the transformation between the Info object & this data source.
3) Create a flat file and upload it.
Hope this helps.