Questions about lme4 package

Dear there,

Thank you for reading my email. My name is Bingsong Zhang, a third year PhD student of Biostatistics at Georgetown University. 

I am doing my thesis on generalized mixed model, which may rely on lme4 package. I know from the documentation that current version lme4 is not allow for specifying flexible correlation structure, so I am wondering if it possible to specify some popular structures like those available in nlmn? And how possible it is if I want to improve based on the current lme4?

Best,
Bingsong