I'm not sure you need to.  The drive letter of my main runtime, has changed a lot lately; do to alternate Boot Drives, USB Pen drives, and a few FDISKs. 

I think Poser will always look for files in whatever Runtime is the Active Runtime, when you open a scene, first; becasue I could never remember needing to keep updating file paths when opening a scene, but this was because I never use any other Runtime besides my External....Until I installed P7, last week; when I forgot to switch from the P7 Runtime, and Poser started lookng for Geometries and Textures, that I'd used a million times.  Then I realized what runtime I was in, switched, and the scenes opened fine!