Adding extra Z-offset to bed for PETG in Slic3r
An extra gap of 0.2 mm on top of your existing gap seems rather illogical as the filament is not squished at the build plate anymore. Furthermore 0.2 mm plus something you already have will soon result in a gap over 75% of your nozzle diameter (including your first layer; note that 75% is considered to be the max for good adhesion). The filament will then drop down to the plate... Possible effects/results can be seen in this recent question.

Having printed about 8 kg's of PETG, I have never had to add an additional gap. I have printed on various machines; Prusa clone, Ultimaker 3E and self build CoreXY. I use a 0.2 to 0.25 mm first layer thickness (for a 0.4 mm nozzle) while printing on glass or directly onto the aluminium bed using a PVA based spray glue (3DLAC). Beds are always leveled with the nozzle to bed distance of a sheet A4 printing paper. I have zero adhesion problems.

I would recommend you to try print as you would normally do and, when that does not work, increase the first layer in your slicer slightly. Printing temperature/speed towers is also recommended to find the best settings for your brand of PETG.

Now to address your specific question how to change the offset in your slicer; an additional offset can be added into your start code script by adding a move to a certain height and redefining the zero Z level. This should be typically done after auto leveling of the bed (if used). So before the first layer is being printed you could add:

; start code contains homing, pre-heating, auto-leveling, nozzle priming, etc.
G1 Z0.2 F500 ; Go to the level of 0.2 mm + your paper thickness
G92 Z0       ; This redefines the zero Z level


You can create custom startcode profiles in Slic3r (requires setting the Printer Settings) and store them with a specific name so you can have different printer profiles.