Force a split in Rpart3

[lsdst8+@pitt.edu]

I would like to force a specific variable to always be the first split 
criterion in rpart3, somewhat analagous to forcing a variable into a stepwise 
regression. I have not come across this in any of the literature (I might have 
just missed it). Can this be done? If so what steps do I have to take? Thanks

Scott