Including an offset variable [or constraining a coefficient to 1] using mlogit R

Question

I´ve been estimating some MNL models in R using mlogit. The package works very well but it seems that it does not allow to include offset variables. I read the package documentation in order to see whether it allowed to constrain a coefficient when it estimated the model. It seems that there is one alternative using mlogit.optim. Unfortunately, it does not specify how it must be used.
So, my point is: does any of you know how to either (a) including an offset variable or (b) how to constrain a coefficient (coeff A = 1) using mlogit? Thanks in advance! Best, Dr. Wall