It will be easy.
You can still use the existing ground wires that currently service the outer roof markers - that will not affect the operation of the inner markers.
For the power to the outer markers - things are going to be different than wiring up a regular directional/parking lamp. The park/blinkers in the grill are dual filament and the roof markers are only single filament.
When the blinkers are activated, the roof lights will go completely on/off (just like the grille mounted lamps do when used without the headlights - like in the daytime).
This is unavoidable - unless you can source dual filament lamp bases that will fit in the marker housings.
Fishing the power feeds up into the void space should be easy as well. Remove markers and you'll have plenty of access to run the new leads across the roof from the LH side over to the right.
Disco any wire that connects to the LH outer marker, tape the ends of the new leads to it and pull them down into the cab. Leave yourself plenty of extra new wire and when they get down into the cab release the tape from the end.
Reattach the original factory wire (that you used to pull the new wires) to some point way up on the new leads and haul them both back up into the void space.
Separate the two wiring groups and attach to the appropriate lamps.
I am not very articulate tonight- but you can probably figure out what I am babbling about.
To get a directional signal, use either the light/dark blue wires that feed the front blinkers or it might be better to use the yellow/dark green wires that feed the rear blinkers.
And that's it - I'm done typing for the night!