Not going to happen sadly - the PCMCIA slot needs to be CardBus compliant, which the 5x0 series does not have. There does not exist a PCMCIA non-cardbus USB card (CardBus is basically a variant of a PCI slot).
You can however use something like a PCMCIA --> CF adapter quite successfully in a 540c. CF-->SD card adapters also exist, include a SD-->MicroSD adapter--->mini USB Micro SD adapter/dongle, kind of like a flash drive.
You can however use something like a PCMCIA --> CF adapter quite successfully in a 540c. CF-->SD card adapters also exist, include a SD-->MicroSD adapter--->mini USB Micro SD adapter/dongle, kind of like a flash drive.