How would one express pemoline in terms of phenethylamine?
I would like to know what its chemical name would be in terms of its phenethylamine backbone.


Artificial numbering and naming schemes that only exist in ones own cloud cuckoo land are pointless. 

IUPAC nomenclature exists for a reason.

Besides the IUPAC nomenclature, other naming schemes - based on established (trivial) names exists and are widely used - for good reason and with acceptance by the scientific community. Typical examples are found in the field of natural products chemistry (steroids, alkaloids, etc.) 

It is however crucial to understand that in these cases, prefixes and/or suffixes denote a variation of the underlying core structure, such as 


additional substituents
lack of methyl groups
an elongated side chain, etc.


In the current case, the central structure is a completely different heterocyclus, so markedly different from the original phenethylamine that it would not make any sense to apply a phenethylamine-based name here. 

Neither would one try to apply such a naming scheme to the morphine skeleton - despite the fact that one might spot a six-membered ring with $\ce{CH2-CH2-NCH3}$ side chain here. 

This does not mean that a member of the phenethylamine family is not among the ancestors of morphine in the biosynthesis! In fact, reticuline, a morphine precursor, most likely is a grandchild of dopamine.

But again, at a certain point structures have become so markedly different that a valid nomenclature (= valid for a particular compound class) no longer is applicable. 

Trying to describe similarities between structures is not wrong, but trying to apply an inappropriate naming scheme is not helpful here.

Similarities between molecular entities should be expressed otherwise.
For sequences (DNA, RNA, etc.), Hamming and Levenshtein distances can be used. For the title molecule and the whole dopamine/amphetamine class, I'd consider the comparison of their fingerprints (Tanimoto coefficients).