It seems that SMPP is recognized by a specified port (even if it's not globally known).
As it says in the Wireshark page:
You cannot directly filter SMPP protocols while capturing. However, if you know the TCP port used (see above), you can filter on that one.
I don't know this protocol well, if you know it better and know how to recognize whether a packet is SMPP or not, we can figure out how to do it in Pcap.Net.
I don't know how Wireshark filters SMPP in the display filter, maybe you can find out how Wireshark does it.