specifying values in correlation matrix in nlme

Could anyone help with a linear mixed model fitting problem ?

The model is : 

Y= Xp + Zu + e 
where X, Z are known design matrix, p is fixed effect factor, u is
random effect,  u~ (0, G) , e~(0,R)

The main problem is , I want to fix the covariance matrix G to be a
constant times a known covariance matrix A,   G = c*A (c is positive
constant, A is a predefined matrix with values manually set by me.

I know the correlation option in lme function can specify some kind of
correlation. but only with the Construct function defined, not whatever
ever form I want.

Any good ideas of how to do this in R ?

Thanks a lot in advance,

Keyan Zhao
Computational Biology and Bioinformatics program 
Univ of Southern California
Email: kzhao at usc.edu