If a commercially available RF jammer has a built-in PLL system, with a bit of hardware/software hacking you -should- be able to reset the frequency range (provided that the actual VCO in the device supports that range). However, if the device is built to block cell phones and only cell phones, I think the only option you have is to replace the VCO with one of the right frequency.
Here's a table of different VCO's offered by MiniCircuits with their frequency range:
Also, the generic frequencies for RFID are:
125 - 134 kHz
UHF (400 – 930 MHz)
So your best bet (if you want to block all RFID) is to either build a Wave Bubble or find another RF jammer that includes a PLL that you can use to set the range of the VCOs.