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

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