It's entirely possible that the SIM card slot is part of the basic chassis for that laptop - if that's the case, all of that model would have the slot, even if the supporting hardware wasn't spec'ed at build time.
Can you install the hardware? Most likely, yes (it's probably a mini-PCI or mini-PCIe card); the question is if you can get the thing.
But, it looks like you can, though - I did find it listed on the HP parts page
for your laptop.
The HSPA card is $228. The SIM card board (I'm assuming that the board itself isn't there, and you have only the slot) is $93.10.
You'll likely need to tear down the laptop to see if you really do have the SIM board in there.