I don't fully follow your question. In any event, I am offering an answer based
on what I think you are asking. The suggested answer is only an adhoc process
and I don't have personal experience of using this technique.
I have heard of some cases where seasonal indices of one series are used as a
proxy for seasonal indices of other series, presumably because of the lack of sufficient
data in the other series. Of course, this would make sense only if the two series
are similar in some sense, e.g., monthly sales of an item of two different brands. Even
in such cases, because of the differences in the scales of mean patterns of the
two series it is important to model the series in the log scale (multiplicative form in
the original scale)
so that the seasonal indices have the interpretation of percentage change. Assuming that
all these conditions are met, you could use this strategy as follows:
1. Fit a seasonal model to the log-transformed version of series 1 (the
one that has sufficient data). Read off the smoothed seasonal component (the s_season
column) from the OUTFOR= data set in the FORECAST statement.
2. Fit a non-seasonal model to the log-transformed version of series 2. Add the
smoothed seasonal component of the first series to the forecasts of the second series.
This would adjust the forecasts of the second series according to the seasonal
pattern of the first series.
Hope this helps.