[R] Using item difficulties from a fitted Partial Credit Model to predict person abilities in an extended dataset?

[Rob Forsyth]

I am using the eRm package to examine the properties of a clinical rating scale using a Partial Credit Model (PCM). I understand how to extract the person ability estimates (thetas) from a simple fitted PCM but I have a dataset with repeated observations over time (~1200 observations of the instrument in ~250 individuals). So as not to violate assumptions of conditional independence I've fitted the PCM to single observations drawn at random from each subject. This works but I would now like to use the item diffculty estimates from the fitted PCM to generate person-ability estimates for the remaining ~950 observations (at other timepoints) not used to fit the model; and I can't see from the eRm package documentation how to do this?

Advice very much appreciated
Rob