Well, the Tecra R850 PT520A-01T00301 supports 3G broadband modem.
The point is that the SIM card slot is part of the motherboard and if your notebook does not support the SIM card slot, the upgrade would not be possible.
The sim card slot should be placed behind the battery…so you need to remove the battery firstly to check this.
The 3G card the PCI-E mini card can be connected to motherboard and its removable.
Furthermore the 3G modules needs an 3G antenna… so the whole notebook needs to be disassembled to install and to hide the 3G antenna cables…
The whole procedure will be really, really difficult and I think only an authorized service technician with good notebook experience could do that.
Well…Paolo was right…the notebook needs to be disassembled…3G antenna cable needs to be installed…this is a job for a technician…without having an experience in notebook disassembling there is a high risk of hardware damage