simulate a time series with knowing parameters

how can I simulate a time series, take a mean from each simulation model to build a new time series. It should be routine work but, however..

My first task is to produce a model for the observed time series and to fit them. So I maked it in excel

Then I want to simulate this model 10000 times and for each model take an average in order to produce a new time series composed of this 10000 averages.

my be there is a toolbox or some scripts for those issues