Is it possible to do ImportRange in Google Apps Script?

Yes, this is perfectly possible. You just need to call SpreadsheetApp.openById and then get the desired sheet and ranges normally.

Please take a look at the documentation : range.getValues() and range.setValues() are very basic GAS methods and are pretty well described.

Read the tutorial as well.

It seems Google, in their infinite wisdom, has altered the behavior of openById and similar functions. They are no longer allowed in the context of custom functions.

See https://code.google.com/p/google-apps-script-issues/issues/detail?id=5174 for more details.

They suggest using IMPORTRANGE as a workaround, but as previously mentioned, this needs to be called within a cell.

Our solution was to use IMPORTRANGE in the sheet, and pass the acquired data into our custom function, as the data set was small. I hope this information helps!

I needed to do this recently. This is what I came up with, simply hard-coding the spreadsheet key and range into ahab's myImportRange function

// to be used in the spreadsheet like so:  = myScriptedImportRange( GoogleClock() )
// no need to include key or range because they are in the script here
// 
// the third parameter - GoogleClock() - triggers an automatic update every minute.
// updated 2011-07-17 (ahab): better regex to strip sheetname of *outer* single quotes
// updated 2013-01-27 (ben) to hard-code key and range 
function myScriptedImportRange(  ) { 
 var key = "PUT YOUR DATA_SPREADSHEET_ID IN HERE"
 var sheetrange = "PUT YOUR SHEET AND CELL RANGE IN HERE"
 var shra = sheetrange.split("!") ;
 if (shra.length==1) shra[1]=shra[0], shra[0]="";  

 var sheetstring = shra[0].replace( /^'(.*)'$/g , "$1") // was: replace( /'/g , "") ; updated 2011-07-17 (ahab)
 var rangestring = shra[1] 

 var source = SpreadsheetApp.openById( key )    
 if ( sheetstring.length==0 ) sheet = source.getSheets()[0] ;
 else sheet = source.getSheetByName( sheetstring ) ;

 return  sheet.getRange( rangestring ).getValues(); 
}

In my case I have a set of private sheets, an intermediate sheet that uses the regular myImportRange and VMERGE with some SQL to combine selections from the private sheets into the intermediate sheet, and then a public sheet that simply has one cell containing = myScriptedImportRange( GoogleClock() )

Note that there is a similar approach here: https://stackoverflow.com/a/11857014

Note also that the ImportRange function and related functions often have a problem of not displaying the imported data when the origin workbook(s) is/are not open. A simple way around this has been described in a comment here: https://stackoverflow.com/a/11786797