How do you Benzylate (add a benzene group to) cyclic compounds?
Brenton, the term "benzene group" is a bit misleading; it seems that you'd like to add a benzyl group, i.e. $\ce{C6H5-CH2}$. 

Typically, these reaction are carried out using freshly distilled benzyl halides. In the case of the diamines, monoalkylation is usually achieved using the bis-hydrochloride of the amine and a stochiometric amount of base, thus keeping one nitrogen atom protected.

Note however, that the Australian regulations, kindly mentioned by Richard Terrett, are likewise true in the EC: 1-benzylpiperazine, an infamous psychoactive party drug, is a controlled substance!