Arduino GPRS (GSM) shield (SIM900) no long pins
Yes, you can.

There is two methods of making shields stackable:


Use the quite hard to source extra long pinned headers to go right through and down (such as these https://www.adafruit.com/products/85) , or
Use separate headers and pins and (if you are using through hole) offsetting them.


Your shield uses the second method, but the descending pins haven't been populated.  By populating those pins you are just completing the build.