Your question is how to add a lag as an input variable? I don't think any of the Modify nodes have a simple way to do it. But it is really easy to do in SAS Code.
In Enterprise Guide add the below when preparing your data, or use a SAS Code Node in Enterprise Miner.
EG (add this statement in any data step, and change var1 for the name of your input variable)
lag_var1= lag(var1);
EM, in a SAS Code Node (change var1 for the name of your variable)
data &em_export_train;
set &em_import_data;
lag_var1= lag(var1);
run;
I hope it helps,
Miguel