[R] How to extend an existing linear mixed effects model using lme in R?

[urs]

Dear list,

I fitted a linear mixed effects model using lme in R on a relatively 
large data set. Now I want to extend this model, introducing an 
additional fixed effect on a subset of the data without altering 
coefficients estimated on the original dataset. I.e.

original model for example:
m1 = lme(y~a+b,random=~1|id,data=D)

now I want to do something similiar to
m2 = update(m1,~.+c,data=d)

Where d is a subset of D. The values of the new predictor c are only 
available for this subset. However, I would like to keep the originally 
estimated coefficients of the model m1 with regard to predictors a and 
b. If I use update as described above coefficients for all predictors 
(a,b,c) are estimated again on the smaller dataset d. Any suggestions on 
how I can estimate the effect of c while keeping the old coefficient 
values for a and b? Or is this for some reason a bad idea altogether?

Urs