Docking at supply ships

Ahoi!
Stiebler and h.sie have a U-Tanker Mod with which you can 'pseudo-dock' at a U-tanker mid-ocean without finishing your current patrol. In the readme file of this mod it says I can dock this way at surface supply ships too after adding a letter 'U' before their names in the campaign file e.g. "Belchen" into "U-Belchen. My question would be how can I carry it out, namely how to properly rename a unit in the campaign file? Thanks in advance!