Hi Andrew!
I´m not sure if you can explicit just load one branch, but you have some options both in transformation file and in Conversion file.
If that dont work then I guess one option is to load the whole hierarchy and then build your pown customized hiearchy and use that in the planning sheets.
For ex if you load the whole hierarchy and then define a Parenth2 hierarchy in BPC and use only Node and Node 2 members in that hierarchy.
Br
Patrick