Dear Elisa, there's no way right now to automatically inherit attributes, so you have to re-add them.
There's a bug in the read-only position inspector, unfortunately, which is fixed in the upcoming 0.8.0. If possible, you can use a read-write inspector, or indeed write a new one. Maybe the simplest option is to have three read-only length attributes?
SB_ATTRIBUTE_READ_ONLY(SBQuantity::length const&, MyAtomClass, X, "X", "Properties");
SB_ATTRIBUTE_READ_ONLY(SBQuantity::length const&, MyAtomClass, Y, "Y", "Properties");
SB_ATTRIBUTE_READ_ONLY(SBQuantity::length const&, MyAtomClass, Z, "Z", "Properties");
I will add a tutorial about writing an inspector and I will let you know about it.