You have given no information about the device you are asking about. You have not specified if the reason for tapering your track to match the pad width is: 1. purely for width matching or;  2. there is a requirement to try to match the 50 Ohm trace impedance to the specified impedance of the device pin. Therefore this answer is based on the assumption that the requirement is for (1). There is no built-in way to taper a track width. You could do it by drawing a track from the pad to some distance away from it and then over-drawing short lengths of track at increasing widths to create a rounded, stepped trace width up to the 50 Ohm trace width. This will be tedious if it has to be done for several pins but the structure could be copied and pasted, taking care to edit the net names of each copied instance. If it is required for more than one pin to track transition then a better solution might be to create a tapered shape using the Solid Region tool and then copy and paste that, again editing the net name for each instance. Better still, create a dedicated taper transition Footprint made as a single pad with a single pad number of 1, shaped as a polygon of the required dimensions. Save it with a unique name and then create a dedicated taper transition Symbol with the same name and assign the taper transition footprint to it. Then place the symbol anywhere it is required in the schematic. When you do Convert to PCB, Update PCB... or Import Changes... the taper transition footprint will then be pulled into the PCB wherever it is required.